It is unable to place any items on Natural Altar except blocks like sand (for making soulsand via Transmutation) or glowstone (for making glowstone dust via Crumbling). Iron ingot for basic Infusing to Infused Iron can be placed without any problems.

Oof, this is a fun issue.
You can still input them using a hopper though, so you can do that as a workaround for the time being.

Note for myself: This happens because the list of catalysts around the altar is only initialized on the server, but is required to find out if items can be placed on the altar.