Starting in 1.21, when you place water or lava while moving with wasd or moving your head around, you see what I call a ghost water/lava (water at the place you ained at when right clicking which then disappears in the next tick). A tick or so later it will then actually place, but at the position you aimed at just ticks later. This is incredibly annoying when you fight in a gamemode like uhc, which is based off of early game items like lava to put the opponent on fire and water to put out the fire or escape cobwebs. Oftentimes you will die to this glitch as you try to sprint hit away the opponent after placing your water to escape a cobweb, but it lands somewhere else and doesn't free you. 

Another Issue which is also oftentimes found in uhc fights is when you hotkey to your empty bucket to take water or lava from the ground and then quickly switch back to your sword, it will very often not actually take the lava or water, but still play the water/lava sound. 

I have reported this issue to the official Minecraft Bug Tracker, where the mods told me after a lot of discussion, that it has to be a spigot bug and I should take care of it myself by writing the server owner (pvplegacy, mcpvp,...) or the people that built spigot. So here I am. If you guys should need any more details from me, please tell.