Initially seen with an upgraded (1.12.2 to 1.3 world), then duplicated with a newly created world. All goes well for a while, then suddenly I can neither break nor place any blocks. I can at that point still interact with bocks such as chests, doors, furnaces, and can hit mobs with a sword.

Things that seem to be realted: the visible collision box on the blocks vanishes when this happens. Also, going into "open to LAN" I find cheats are off (both worlds created with them on). I tried setting it to on, and open to lan... going back in they are shown as off. NOTE: the select world screen shows cheats on.

It feels like I'm in a partial-observer mode... but cannot walk through blocks.

 I tried changing gamemode - in creative, I can break and place blocks, but when I return to survival it is still broken.