There is a large thread in the Google Analytics Help Forum with complaints from site owners of being flooded with fake referral traffic originating from Poland. This seems to be the typical referral spam issue and it started around February 17th or so.

There are actually several threads with complaints around this issue in the forums, here is a list of those complaint threads:

Referral Spam news grets store (Poland)
Traffic from Warsaw Poland.
Referral Spam from news.grets.store (Poland)
unexpected increase in traffic from Poland
Poland traffic out of control
Fake traffic hits from Poland are skewing results.

Those are just some of the complaint threads on the topic.

There are literally over a thousand posts from GA users with complaints about this referral spam issue in Google Analytics.

Here is one screenshot showing how real time data for one site is showing this explosion of traffic from Poland:

This shows how the referral spam is showing this traffic as real:

Here are some screenshots showing the source of the traffic; many from Russian domain names:

I do not see any response from a Google representative on this issue yet.

As far as I can see, there is also no clear way to filter out this traffic.

Forum discussion at Google Analytics Help Forum.