With a specific setup, a survival player can become softlocked in a hole with a sulfur cube

When a survival player falls in a 1x1 hole with a sulfur cube at their head height, then unless they have ender pearls or are able to jump-break one of the enclosing blocks, they are stuck and must wait to die by starvation (if the difficulty allows, and there is no Regeneration beacon nearby) or, most commonly, open to LAN with commands enabled or ask a server operator to free them. This is not usually an issue because invulnerable entities of sufficient size to trap the player in this way have historically not existed; assuming none were summoned with Invulnerable:1b, only creakings are truly invulnerable, and the fix for MC-277123 means that they cannot softlock the player any more. However, sulfur cubes with a block inside can still trap the player in this way (assuming the player does not have shears or an empty bucket), although due to their dimensions and lack of hostility it requires a setup specifically tailored for doing so. See the video below for a demonstration.

Since the setup requires a block appearing above the player after they have fallen into the hole, the risk that this would randomly happen to anyone is quite small, and most likely this interaction would mainly serve as a way for players to troll their AFK server-mates. In a UHC-like situation, it may be possible to sneak up on a player in a 1x1 hole, place a trapdoor at their feet, unbucket a sulfur cube at their head, place a block above them, deal with their teammates and finally kill them by attacking their legs, but even this would rely on them not having a bucket, shears or two iron ingots; dropping TNT into the hole, or just attacking them in melee, is much more straightforward. All the same, I figured Mojang should be aware of the issue before the drop releases, since if they do consider this a bug (and worth fixing) then now is the best time for them to address it.