I just tried this on another French forum - which was not imported - and topics from around that time do indeed not contain accented characters in their search excerpts.
Weird is that when I look at German forums, old topics do contain accented characters in their search excerpts 
Look at the last line of this log
commit cee51672c96d29174196717b734c7f7ba0754172
Author: Daniel Hollas <danekhollas@gmail.com>
Date: Tue Oct 23 03:10:33 2018 +0200
FIX: Strip accents from search query
4481836 introduced accent stipping in search_indexer,
but we need to strip it from the query itself as well
TODO in search with diacritics:
- Still need to fix excerpts on search page