There are two mods fixed MC-73186 in 1.16 and above version: Item Stitching Fix and Item Model Fix (Fabric).
They fixed those transparent lines in all generated item models, but no mod as the same action in MC 1.12.2, can you have a look?

Model Gap Fix is also worth checking out. It fixes that bug too except it also expands the fix to block models and anything else with similar functions.

I was indeed looking at Model Gap Fix for the most benefit and already have a rough idea how to port it.

Implemented in b9db738.