Yes. It does happen again, if the cage is 3x3
I could fix it by adding a Scoreboard team with collison rule Never.
=> Mobs can glitch out of a box, if they are to "fast".
Screenshots of it
There are those 2 things
after Server restart it was normal again
EDIT:
I reuploaded the Map and it was there again..
Restart helped. Maybe a Bug with first time server loading the Map
Dropping: http://hastebin.com/elurumiwut.coffee
Picking it up: http://hastebin.com/dusizaguhe.coffee
Server IP: niklask2001.de:25564
There are no errors in the Console / ServerLog
On what? putting it onto the ground or picking it up?
This also happened to me and it looks like going into fullscreen and exiting fullscreen mode (pressing f11 twice) fixes the issue