Bucket Placed Fish Despawn from Aquarium

From reading closed support calls it seems fish placed from buckets into an aquarium shouldn't despawn, however with 1.13.0.1 Beta release the 10+ fish I'd placed from buckets into an aquarium I've built have all disappeared (Survival Mode).

I want to confirm that bucket placed fish should be permanent. Am willing to accept that maybe these fish have all been killed somehow, but I think I would have noticed if that was the case. Do they get killed by lightening perhaps?

Someone mentioned you can rename fish (in their bucket) on an anvil with a nametag, and this should also make sure they don't despawn. Tried this with a bucket of red snapper and couldn't rename... Does this only work with certain fish?