There has always been steady flow of Polish migrant workers to and from the UK. Workers would make money and develop skills in the UK, sometimes remaining permanently but also often returning after a number of years, particularly once you get you the “raising a family” stage in life where lower cost of living and being close to extended family becomes a bigger attraction.

The difference now is that the supply of new workers has been reduced by Brexit, so only those existing workers with residency rights remain. I doubt there has been any significant increase in the number of workers leaving, aside from temporary changes during the pandemic.