The Miyuki anime has a lot of problems. It messes up the timing of events, adds in filler padding into the middle of things that did happen, cuts some things that happened, changes events from summer to winter, and so on. These minor issues can be overlooked to some extent, but what's really unforgivable are the two remaining issues.
The first problem is that the Miyuki anime ends around the 51st chapter of the manga. After that it spottily adapts a few more chapters up until chapter 60 of the manga, after which it goes entirely silent. That leaves at least 35 chapters left to adapt before the true ending of the series, which is a very excellently told ending we've all been waiting for, the happy marriage of Masato with his step-imouto Miyuki. Leaving out the most important part of a romance story, the confession and the wedding, is ridiculous. What kind of love story doesn't animate the best part?
The second unforgivable problem is that they wasted time on completely filler anime episodes, made up junk that was not in the manga whatsoever, when they could have been adapting more of the manga which they instead chose to never adapt at all.
In total, five episodes of Miyuki are filler and therefore should not be watched at all, in order to get the correct 'feel' and 'mood' from the series, which can only come when you stay on target with the author's original intent. These five episodes are:
15, 16, 22, 26, and 33.
When you watch Miyuki, be sure not to watch these five episodes. Not only are they all terrible episodes to begin with, they're also false episodes which portray the characters and their relationships incorrectly, which is never a good thing for a people driven story.
This means the Miyuki anime is in truth only 32 episodes long. In addition, it only covers around 1/2 of the manga source material. It's fine to watch the Miyuki anime first, but it's still necessary to read the Miyuki manga afterwards. Otherwise you'll just never understand how great Mitsuru Adachi really is. Miyuki won a prize for best manga when it first came out. This is seriously high caliber story writing here. The anime doesn't do it nearly enough justice. Along with H2, they should just make a sequel season already and get it right this time. . .