To reproduce,

1. Starting with Minecraft 1.16.5, install Fabric loader and Fabric API.
2. Install mobdisguises-1.0.0-fabric and disguiselib-1.0.2-fabric.
3. Create a new world with cheats enabled.
4. Find or summon a few sheep.
5. For the base case, give yourself a lead and leash one of the sheep. Notice that that should succeed.
6. For the problematic case, run /disguise @e[type=sheep] as creeper, then give yourself another lead and try to leash another one of the (disguised) sheep. Notice that that leash fails.
7. /give @p lead 64
8. /gamemode survival @s
9. Repeat steps 5-6. Notice that the leash pops off immediately instead of you not being able to leash the entity at all.