I haven't done a lot of in depth testing of this bug from what I know right now, placing magma blocks (maybe in a certain way) and then putting water on it can ruin the physics of water in minecraft. I first noticed this when I built Ilmango's mob farm and a creeper blew up some of the magma blocks I replaced them and now when water interacts with the affected blocks it will not spread properly, and then not drain.
Thank you for your report!
We're tracking this issue in MC-42387, so this ticket is being resolved and linked as a duplicate.
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