Command Blocks not working properly

Command blocks cannot summon zombies or remove xp levels. Which they could both do in previous versions. I use both these features in my server so I'd really hope they'd be fixed. I usually use /xp -100l @a to keep all players at 0 xp because im making a tier system and the number of the xp bar is important but it says "cannot give negative xp". Is this a removed feature? New command? Or just a bug. The zombie one is definitely a bug though - pilligers and piglen still work.