We recently migrated and then upgraded a MediaWiki, and the upgrade was a major upgrade.
It run successfully, but then tome pages, such as the home page, showed errors like this:
MediaWiki\Revision\RevisionAccessException from line 1676 of /home/mysite/www/www/includes/Revision/RevisionStore.php: Main slot of revision 38921 not found in database!
We found several reports of this error online and the Mediawiki RevisionAccessException forum page on https://www.mediawiki.org/ suggests a solution. We tried that and found that using
$wgMultiContentRevisionSchemaMigrationStage = SCHEMA_COMPAT_WRITE_BOTH | SCHEMA_COMPAT_READ_OLD;
in LocalSettings.php fixed the problem.