The /tp command causes an entity mounted on another to dismount without the use of the /ride command. In previous versions the entity sat despite the teleport command. I used /tp to rotate the entity and noticed that it would now dismount.
Commands to reproduce:
/summon zombie ~ ~ ~ {Tags:["1"],Passengers:[{id:"minecraft:zombie",Tags:["2"]}]}
/execute as @e[tag=2] at @s facing entity @p feet run tp @s ~ ~ ~ ~ ~