I'd read 'temporary work assignment' as being more accurately 'undocumented worker without status'.
Yes, I know that officially there's around a million 'legal' Ukrainians in Poland but the real numbers are way higher and they are not there for the sunshine and beaches!
Most of the legal Ukrainian workers in Poland stay for about 3 or 4 months. It'd be hard to describe such workers, whether legal or not as being on an assignment. Can you imagine a domestic servant being so influential and unique that her services were in demand for short term international duties away from her work in Ukraine? To be honest, the same applies to accounting!
If these women are not from Ukraine but from Belarus or Russia then the chances of them being legal are much, much, lower than if they were Ukrainian.
So, to directly answer your question: No, these assignments are not a common thing. You are almost certainly being misled either by an accident of language or more directly. You might want to ask some friendly but direct questions about what your interlocutor is really up to and remember that gut instinct is very often a good instinct.