Over in #1314214: MySQL driver does not support full UTF-8 (emojis, asian symbols, mathematical symbols) → , indexes that were going to exceed the maximum length allowed were automatically truncated. This change was only applied to normal indexes though, and not unique or primary keys.
Shorten primary and unique keys as needed.
Closed: works as designed
9.5
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It's likely this issue predates Contrib.social: some issue and comment data are missing.