Hey this bug has been in Minecraft PE/BE for years. So basically in your first slot put a sword and in your second slot put a block or item in it. Hold your sword and start hitting an entity really fast maybe like 12 clicks per second) then switch to your 2nd slot that has a splash potion while you're still clicking. It glitches out for some reason. It takes a couple tries usually
(you can use an autoclicker to reproduce this better)