As part of the production migration to PHP 7.4 (sharing some caches with PHP 7.2), we patched our PHP 7.4 to emit 7.2-compatible serialization.
This worked, but broke some tests that expected PHP 7.4 to emit the new serialization.
Once the migration to PHP 7.4 in production is finished (T271736 or T318894), we should remove this patch again, and use the new serialization.