How does Hallmark guarantee such consistency between their Christmas movies?

I'm going to drop this into the answer space as a temporary space-filler rather than as a final product. I don't have the time to fully flesh it at the moment, so it's open season for anyone who wants to fill this answer, or just go for it and write your own.

I can't answer for this franchise specifically, but many many TV shows use a similar 'look book', a house-style that is not to be broken. Few shows these days have a 'one director for the duration' policy; they instead have a rule book and more importantly a show-runner to oversee the overall look & feel.

I want to add this as input, but not as a final answer. I don't know anything about what Hallmark's internal policies might be. But, part of the reason this happens is because "Christmas movie" is such a narrow genre. It's like shooting a zombie movie, there's really not too many directions to take it and run.

If you watch almost any Christmas movie, it will have their characters experiencing the "magic of Christmas" in some way. Possibly a love story intermixed in there. You have extremely common tropes, such as:

• The scrooge - the character who HATES Christmas, until (event) when he practically becomes St. Nick.
• A scene about setting up the Christmas tree, hanging lights, etc.
• Some adversity that will cancel Christmas, which might be resolved by some "Christmas miracle"
• The character who is WAY over-enthusiastic about Christmas. This is the opposite of the "scrooge" troupe. Think: Elf

In all of this you wind up with very common framing for some of the shots. Generalizing, you wind up with outdoor shots of a house covered in lights and snow. Shots of a decorated tree, usually with a fireplace for some reason. Scenes of a family eating Christmas dinner (that one varies depending on the 'type' of Christmas movie).

So, there very well may be a tight handbook that Hallmark specifically has to follow. But the genre itself is very prone to repetition.

In addition to tetsujin's answer, I'ld like to give an idea of those style guides. These are given to different teams involved in the creation process to allow for consistency of viewer experience.

Here is an example of the "look book" for King of the Hill. This allows for a wide variety of animation teams to be brought on as needed, while still giving the exact same King of the Hill experience to viewers every time.

I will focus on technology.
Those movies look so similar because they are made for specific "target". Hallmark might (and probably is) using a set type of camera filters. To make a cookie-cutter film you need to make a cutter. One of the requirment is pipeling the production time and effinency. So now an editor will have a set of presets for color, balance, light corrections. And they will make all movies look similar.
On-set lighting. As you might notice all morning/breakfast shows have similar lighting. And all of them use "old type" big cameras (with a rigs or handhelds as novelty). The light are set in particular way for those cameras. They enforce the amount of light that is needed.
It also cut extra time (and people, no gaffer needed) who normally would work on lighting (for more of that please refer to this instructions and explanation Vanity Fair what Gaffer do ).

So instead of having A star with different script, scenery, mood or even genre of movie you just swipe the star in well establish place and you can roll.

I don't know how many movies (other than Christmas) Hallmark produce each year but just a quick look at their Christmas portfolio show a gigantic amount of money they need to spend. Cutting down the cost and speeding up the production (while maintaining same level of quality) require some pre-existing processes to be created and enforced.

There's an old musical equivalent to this called a "Sonata Factory". Basically, you write and publish a piano sonata (can work for other instrumentations, but let's just stick to that for simplicity), then you take that sonata, and rewrite a new part for the right hand to go with its left hand part, and then rewrite a new part for the left hand to go with your rewritten right hand part, maybe transpose the whole thing to a different key, publish that, and repeat. (You only publish every other piece or it would be obvious that 2 sonatas share the same part on one hand.) What you wind up with is a whole bunch of piano sonatas that are all the same length and have the same general structure and feel, but are superficially different.

The same model can be applied to movies, particularly movies in as narrowly defined a genre as "Hallmark Christmas movies". You get one, change all the characters but leave the plot the same, then change the plot points but leave the characters, rewrite all the dialogue to fit your new characters and plot, film it, distribute it, and repeat. Many of the other aspects won't change, the setting tends to be the same from one Christmas movie to the next, the basic message of the movie never changes. The audience isn't looking for anything groundbreaking with these movies. They're looking for something familiar and nostalgic, so you can get away with reusing the same elements over and over again, and it's much easier and cheaper than coming up with something truly original every time. Given the limited amount of time they have to make these films (people expect a half a dozen new films to be released every year), it's not surprising that the filmmakers would take shortcuts to churn them out in time for the holiday season.