MM wiki was created years before the migration to the new markup (especially infoboxes templates) and Fandom (2019), so migrating the site (over 1500 articles) poses some risks and is no easy task. The orginators of the MM wiki want to preserve the format and content – the look and feel – , so it is a slow process Admin focuses on (underneath the hood) during the summer before each new season (and articles are added) that is why we've been rather finicky about format, links and syntax. You can see the difference between the templates 'infobox cast' (new) and 'infobox character' (old) as an example.
Very tech-y-geek-y – totally different code language from old to new: Admin is "experimenting" on the live 'infobox cast' template before the other 12 templates will migrate. There are also some quirks in the Navbox and other templates as well; all this has to translate/work on all desktop browsers and mobile devices.
When you are editing do you use the VisualEditor mode or Classic? In the Classic mode you have the Source tab which gives you access to the code syntax (markup) of the article: this is where one can edit the markup. So, I have noticed when one edits the infobox from the VisualEditor mode (clicking directly on the inbox to add information), this can 'scramble' the code syntax into a gibberish-looking paragraph in the Source. In Classic rich-text via the Source tab, one can add information and arrange (type) the paragraph infobox into the line format of the markup.
Any other fixes to the infoboxes is via the original template and deeper under-the-hood stuff is an Admin only access on Fandom.
Also, you can select the Classic mode as a default setting via your account preference.
Did the same when I first started, now I first edit in the Source, then check it out in VisualEditor, and then in Preview; Have to say my skill in code syntax has improved, but my grammar and spelling is another matter xD