This also affected 1.18.12, and probably earlier versions like 1.18.11 and 1.18.10, but my memory is failing me.

A Drowned will pick up any dropped Nautilus Shells they find, which makes sense, and is not the issue. If a Drowned is attacking a player it will completely ignore and will not pick up the Nautilus Shell until the player leaves the area, like exits the water, is killed, etc. That also makes sense, and may or may not be intentional, but is not the issue. It is unknown whether a Drowned attacking a Villager will pick up a Nautilus Shell.

The issue is that the Drowned will pick up the Nautilus Shell even if it has a weapon in its main hand. Rather than putting the Nautilus Shell in its off-hand, where the shell is normally held when it is spawned, it instead is put in its main hand, and any weapon will be dropped. The weapon is dropped with full durability every time, and may have enchantments if they were present.

Due to the fact that a Drowned will burn in the sun, and during the night will attack players, this bug is difficult to reproduce unless in the water. It is also difficult to reproduce this in water deeper than 1 or 2 blocks, since the items will float away. This also seems to occur with only Nautilus Shells, although it possibly occurs with other items.

I discovered this when I was killed by a Drowned in an underwater ruin. I had a Nautilus Shell with me (I probably had several actually) and when I returned to recover my items I found that the Drowned hadn't despawned and had dropped its Trident, still with full durability, to pick up one of my Nautilus Shells. I also apologize if this is a Duplicate Report, but I couldn't find anything else about this.