Drowned can get `DrownedConversionTime` data from other mobs through `/data modify` and will proceed to drown. They will still be Drowned, but their NBT will be gone. Also, unlike with zombies and husks, this command (/execute as @e[type=drowned,nbt=!\{DrownedConversionTime:-1,InWaterTime:-1}] at @s run data merge entity @s {DrownedConversionTime:-1,InWaterTime:-1}) can't fix it.