Qualifications: Server Admin
Server Specs: Fabric, Lithium, Phosphor, and a few VanillaTweaks

After upgrading to 1.16.3 I take fall damage on waterlogged slabs. I went into a test world, also with 1.16.3 and I take no fall damage on waterlogged slabs, so I'm thinking this is related to my fabric server or one of the performance mods. I've tried using different blocks, arranging things differently, logging off, restarting the server, etc.

If this isn't a fabric server specific issue, let me know.

Update:

I removed Lithium and ran the server again. The issue went away. I added Lithium back in, the issue was still gone. So I think this was some situational glitch. I'm happy it's gone for now, hope it doesn't come back.