Biking against traffic on a one-way streets in Europe
In Holland, were I assume cycling is much more common than in Poland, the kind of traffic sign you describe are abundant (see example, "uitgezonderd" is Dutch for "except for").



And cycling against traffic in a one-way street without the sign is indeed illegal. The same rules seem to apply in Poland, though I cannot find a reliable source for Polands cycling laws. As to why Google Maps suggests those routes remains a mystery, but it doesn't seem to happen only in Poland, see this link