If CodeMirror 6 is a whole new system, hopefully that'll resolve the issue. It's not the most annoying thing in the world, but wikilinks within captions are an extremely common occurrence, so it's very widespread.

If CodeMirror 6 is a whole new system, hopefully that'll resolve the issue

I doubt that resolves it, its more that we would have to reimplement it again in that system, and doing it twice is gonna be hard.

No the issue here is more that MediaWiki has a very badly defined set of allowed nesting with lots of side rules where behaviour depends on the surrounding wikitext. Wikitext is notoriously difficult to implement a language parser for, and therefore the CodeMirror highlighter just mostly doesn't deal with any of that nesting, to avoid having to be a full wikitext parser. This explains most of the CodeMirror tickets that are "something wikitext doesn't work inside of [x]".