Summary
When two or more entities ride a mob and the vehicle turns into another mob, only one passenger continues to ride the mob; the other mob gets dismounted.

Steps to Reproduce
Note: reproduction requires commands, as neither Camels nor Boats can be converted into another entity.

1. Summon a Mooshroom
2. Make a Pig ride the mooshroom

   /execute summon pig run ride @s mount @e[type=mooshroom,limit=1]
   

3. Make another mob, Skeleton ride the mooshroom

   /execute summon skeleton run ride @s mount @e[type=mooshroom,limit=1]
   

4. Shear the mooshroom

Expected Result
Both the pig and the skeleton continue to ride the cow (converted from mooshroom).

Actual Result
Skeleton gets dismounted.