AEM 6.3: Alias resolving not working for some pages after content migration (6.1 > 6.3)

We've updated our AEM from 6.1 to 6.3, everything is up and running except our alias resolving.

Things I've noticed:

- /content/site/shoe (sling:alias=foot) --> /content/site/foot does NOT work if 'resource.resolver.optimize.alias.resolution' is enabled in the apache sling resource resolver factory configuration. If we disable the flag, it does work.

- If I update the alias using the advanced tab under properties the new alias get picked up and resolving works

Why do some old migrated pages with an already existing alias fail to work? Any help is greatly appreciated.