I noticed it can also happen with restraints applied by the Dragonslayer sword. It can apply a straitjacket and armbinder at the same time.
Viewing post in Prisoners Spawning with Illegal Combinations
I don't think that armbinder+straitjacket = the same problem.
The game does not adhere to common sense, when dealing with playable character's bindings <-> binding slots, as strictly as it currently does with NPC bindings.
I remember ending up with combos like rigid (hinged or... whatever those rigid handcuffs are called) handcuffs + boxbinder. Which, even for immortals, is impossible without breaking few bones.
Well, that's what Alpha versions are for (<- we are here). To sort out core gameplay.
In regard to this theme's specific problem, boiled down issue description is like "bondage furniture accept characters which already have restraint-forced pose, which is incompatible with said bondage furniture".
Like getting railed by one bar prison trap while wearing a petsuit - I believe that it is still a thing. Which "as currently is" - even for immortal - is supposed to cause... barely compatible with "existing in sound mind" health condition, to say the least. But it doesn't currently.
I believe, the main cause is that "bondage furniture" is a restraint slot on its own (which is PROBABLY shared with big bubbles).