That happened to me once on monstrous specialist maelstrom. I got caught by a spawn and then a plague ogryn came to slam me to death while the spawn was chewing on me.
When a chaos spawn grab you every enemy can still hit you, mostly flamers but can happen with most of them (gunners, reapers, poxbusters, shooters, even melee). This also happen with mutant grab.
Don’t know if all that is really intended or just a big mistake from you know who, but anyway it feels so bad and unfair when it happens.