I think the issue might have been caused by confusion over the difference between coral fans and coral. Brain coral fans, the fan OP described in discord when he noticed this, are able to be placed on the side of slime blocks in the nether fine.
I think the issue might have been caused by confusion over the difference between coral fans and coral. Brain coral fans, the fan OP described in discord when he noticed this, are able to be placed on the side of slime blocks in the nether fine.