Sometimes, after updating modpack to next version, custom tweaked mob drops not working.

For example, after recent updates players found that Zotzpyres not dropping Hardened Leather and Nether Wings.

1. Loot drop for Zotzspire added here.
2. It uses CraftTweaker function addPlayerOnlyDrop.
3. When server is updated, entities numeric ID's are changed, and this function not working anymore.

This is most likely an CraftTweaker bug.

List of entities affected in current modpack state (v1.47.0):

betteranimalsplus:badger
betteranimalsplus:bobbit_worm
betteranimalsplus:feralwolf
betteranimalsplus:fox
betteranimalsplus:goose
betteranimalsplus:horseshoecrab
betteranimalsplus:songbird
betteranimalsplus:tarantula
betteranimalsplus:zotzpyre
biomesoplenty:wasp
botania:pink_wither
botania:pixie
emberroot:deers
emberroot:hero
emberroot:withercat
endreborn:chronologist
endreborn:endlord
minecraft:endermite
netherendingores:netherfish
openblocks:mini_me
plustic:blindbandit
plustic:supremeleader
rats:marbled_cheese_golem
rats:pirat_boat
rats:plague_cloud
thaumcraft:firebat
thaumcraft:thaumslime
thaumicaugmentation:primal_wisp
thaumicwonders:corruption_avatar
twilightforest:deer
twilightforest:harbinger_cube
twilightforest:ice_crystal
twilightforest:pinch_beetle
twilightforest:redcap
twilightforest:redcap_sapper
twilightforest:slime_beetle
twilightforest:squirrel
twilightforest:stable_ice_core
twilightforest:tower_broodling
twilightforest:tower_golem
twilightforest:unstable_ice_core
twilightforest:wraith

Investigation shows that this issue in not reproducible with only CT installed, but can be reproduced with REID.

Is there a solution at the moment? My server is stuck at version 1.46.0 and I'm hesitant to update

There is no known solution for this.
Most drops from those entities is not progression blocking, so it should not interfere to play modpack.

Is there a solution at the moment? My server is stuck at version 1.46.0 and I'm hesitant to update

"If I create a new world on the server, will this problem not occur? After updating, does this issue only affect the old world?"

"If I create a new world on the server, will this problem not occur? After updating, does this issue only affect the old world?"

If you create new world there will no be such issue. This issue happens only after some updates.