Many mobs on realms don't drop items when killed, either by player or not.
Examples: slimes, iron golems, squid, glow squid.
there are probably lots more but I have not tested
Linked issues
Comments 2
Resolving as a duplicate of MC-269887 since identical behavior is described in both reports. If this isn't the cause of your problem, please leave a comment and we can reopen this ticket. Thank you!
(Most likely) duplicate of MC-269887. Please disable any outdated datapacks.