Overridden mobs in old maps can't be summoned anymore

Partner Rank: 1

Verification builds: 1.16.63
Regression builds: 1.16.61

Summary: Old maps that override vanilla mob files can no longer summon these mobs. Seems to affect format_version 1.2 or so, since modern maps are still working fine.

Impact: Almost all really old maps completely broken, unplayable

Repro Steps:

1. Download and open Adventures in Khundalan.
2. Try to /summon zombie

Expected Results: Custom zombie replacement mob is summoned.

Observed Results: Content log complains about tons of functions that try to summon mobs. Syntax error when trying to summon a mob.

Screenshots/Videos attached: No