There might go something wrong with monster's and item's z-height collisions(?). (With Vavoom 1.27.1 too)
If a monster is spawned on top of an other non-active monster they pile up and everything looks normal. But when they both wake up, the upper monster kind of drops inside the lower one and they both are stuck.
A test wad for Doom2 here: For example hit the switch and the shoot one bullet.