I can confirm this issue.
I installed the Korean keyboard and indeed in composing mode (Korean characters are "built" from elements) the text color is white. With ‘Highlight matching tag pairs’ enabled, white text on a grey background is virtually unreadable.
Unfortunately, I do not see any simple solution here. Changing the color for "Line Highlight" is not a solution, since the text can either be white (while typing) or black (after typing). It looks a bit better in dark mode, but not really.
In general, I suspect that the problem is related to the "composition strings".
More:
https://learn.microsoft.com/en-us/wi...osition-string