Open https://www.wikidata.org/wiki/Lexeme:L406433 and watch the senses section. Initially, the name "Jèrriais" is displayed, but then it gets replaced with the code nrf-je.
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The example I gave isn't reproducible now, but the underlying problem is still there, e.g. https://www.wikidata.org/wiki/Lexeme:L406433?uselang=fr loads with "jersiais" which is replaced with "Jèrriais" after loading.
Using the language code qqx instead, it seems it initially loads the message wikibase-lexeme-language-name-nrf-je, and I assume it then replaces the language name with the name from the CLDR extension (LocalNamesFr.php (link) doesn't have nrf-je and would fall back to the English name).
I'm not sure why it's replacing the names at all, rerendering what is already there seems a bit pointless.