The 1.21.90.20 changelog says:
Leashed entities can now be "stolen" by leashing them again, transferring ownership to the player.
However this functionality doesn’t work on cows. If a cow is leashed to another emtity it cannot be “stolen”.
MCPE-217585
It's not possible to re-leash a cow if it's leashed to another entity
Resolved
Cannot Reproduce
0
I can't reproduce this in the current preview. Are you still experiencing this issue?