As of 932f9c3eb, ApiQueryLanguageInfoTest calls ConvertibleTimestamp::setFakeTime in the following way:
$time = 0; ConvertibleTimestamp::setFakeTime( static function () use ( &$time ) { return $time += 0.75; } );
CI does not complain, but ConvertibleTimestamp::setFakeTime clearly documents that the parameter passed to setFakeTime can be "a callback() returning an int representing a UNIX epoch". Up to v4.0.0 of wikimedia/timestamp, ConvertibleTimestamp didn't enforce that documented expectation. In R1985:bbba1bc9ab46: setFakeTime: add $step parameter, @daniel changed MediaWiki-libs-Timestamp in a way that casts the callback's output to an int, breaking ApiQueryLanguageInfoTest.
The magic numbers of 0.75 and 1.5 were introduced by @Lucas_Werkmeister_WMDE in rMW67b3cdc0047f: Add action=query&meta=languageinfo API module (without calling setFakeTime though). In rMW9b41241cc88e: api: Use fake timer to test ApiQueryLanguageinfo, @Umherirrender started using setFakeTime, effectively causing this inconsistency.