"An individual is defined as resident of Poland, if at least one below-mentioned condition is fulfilled:
the individual has closer personal or economic relations with Poland (center of vital interests) or The individual stays on the territory of Poland longer than 183 days in a given fiscal year."
So if you have somebody working abroad, paying tax abroad and sends home money to family, then they are open to being double taxed.
I'm sure there is some better article about all this, but this is what I've been told from Polish nationals a few times and one who was giving up a job in another EU country to move back to Poland, due to this and that opportunities for work had improved greatly from a decade ago when they initially left.