Even the best movies set in the Middle Ages don’t get the period entirely right. Sometimes the production designer doesn’t have the resources or expertise to do the costuming or other details properly, as in the low budget The Seventh Seal and The Navigator: A Medieval Odyssey. Sometimes the film is less about the period than something else. The Seventh Seal was set in the fourteenth century, but Bergman was primarily using the setting to show the sort of existential doubt and crisis of faith a Lutheran pastor’s son might feel in the second half of the twentieth century, with an allegorical personification of death substituting for unfilmable internal conflict. The 1964 Becket was mostly a way to talk about French collaboration during World War II without poking the recent wounds too directly.
Our sense of what is natural, stylish, or flattering is different from theirs, and this often shows up in the recreation of hairstyles and costume details, either unconsciously or because the production designer was afraid the audience wouldn’t accept or understand an accurate recreation. In an extreme case like A Knight’s Tale, the production designer deliberately modernizes the clothing and even soundtrack to make it more accessible to a modern audience even while retaining some properly medieval story elements. The movies can still be worth watching, but you need to be aware of their limitations as a recreation of the historical past.
And then, even if the director has been given an adequate budget, and isn't trying to use the Middle Ages as a way to talk about something else, and hasn't transposed elements of the past to make them more accessible to a modern audience, another challenge remains.
Movies are not made to lose money. Successful movies, like other drama, tell a narrative that makes sense to the audience, and create a plausible narrative arc that ends at the end of the movie with the audience feeling that they have achieved some sense of resolution or release.
Real history, on the other hand, is full of awkward digressions from a plausible narrative arc that ends at with the audience feeling that they have achieved some sense of resolution or release. A successful director will divert your attention from these, or omit them, or sweep them under the rug.
Shakespeare treated history the same way. If you had to choose between historical truth and effective drama, truth would lose.