7.7 changed the object type of string field values that are passed to UpdateRequestProcessor implementations from java.lang.String to ByteArrayUtf8CharSequence.
SOLR-12992 was mentioned on solr-user as cause.
The LangDetectLanguageIdentifierUpdateProcessor still expects String values, does not work for CharSequences, and logs warnings instead. For example:
I'm not sure, but there could be further places where the changed type for string values needs to be handled. (Our custom UpdateRequestProcessor are broken as well since 7.7 and it would be great to have a proper upgrade note as part of the release notes)