Basically every mob drops xp orb right? But sometimes the orbs kinda go underground or become invisible, the issue is that you can see a orb following you but you can't take it because the real one is glitched somewhere, that makes mob farm useless. too often the ender dragon don't drop all of the xp because of this glitch, i have some clips ask me i you want. Sometimes instead of collecting more then 60 levels (on the first ender dragon of course) you only level up to 40 and that's unfair, this happens to the resummened ender dragon too. This happens with everything : every mob, minerals, xp bottles even furnace and trading but it's harder on the last two.