The OwnerUUID tag for pets is a bit bugged in that you can't properly clear it with commands... Attempting to change it to anything other than another proper UUID (or absolutely nothing, e.g. {OwnerUUID:""}) causes the pet to become feral as expected, but the tag refuses to actually update (as well as still showing death messages & being passive to the previous owner). And trying to set it back to the owner causes the command to fail due to it "already having the value". For the record, /data remove doesn't do anything at all to OwnerUUID either.

I'd have thought setting it to be completely blank would work at least, but nope, turns out it unexpectedly doesn't change anything for some reason, despite being what should be one of the most logical way to go about removing ownership. Example:

/data merge entity @e[type=wolf,sort=nearest,limit=1] {OwnerUUID:""}

As mentioned, adding anything in the quotes that doesn't conform to its usual UUID format causes it to become neutral to everyone other than the last owner.