> The Czech layout isn't exactly an obscure edge case.

From what I understand, the problem wasn't with typing characters actually used in the Czech language such as á, ř or ů. The problem was with typing the ˇ character by itself, which is normally encoded in Unicode as U+02C7 (CARON), but there also is a combining version (U+030C, COMBINING CARON), which is what gets printed if there is no precomposed character (e.g. š is both U+0161 and U+0073 U+030C). There is a thing called Unicode normalization that makes "identically looking" strings actually use the same codes, so maybe it was that thing that changed a bit (maybe even somewhere else and not in the lockscreen/keyboard logic), or they could have just removed the ability to type ˇ by itself altogether since it's not something actually used in any language or writing style and most often comes up as a result of a typo.