[...] According to the research, the investigators were apparently able to gather sufficient evidence in recent months to obtain an arrest warrant for the Ukrainian Z. from an investigating judge at the Federal Court of Justice at the beginning of June. In June, German prosecutors are said to have approached the Polish authorities with a European arrest warrant in the hope that the suspect could be arrested. [...]

There has reportedly been no response from the Polish side to the German request for legal assistance. It is not known why the Polish authorities have not arrested Volodymyr Z. According to the common rules of the European Arrest Warrant, which Germany and Poland consider binding, an arrest would have been expected within 60 days without further examination by Poland. The deadline has now expired. [...]

A spokeswoman for the Polish Public Prosecutor General's Office has now confirmed to the news agency dpa that a European arrest warrant issued by the Federal Public Prosecutor's Office for the arrest of a suspect has been received. [...]

German investigators had repeatedly asked their Polish colleagues to hand over CCTV footage from the port of Kołobrzeg. Most recently, the Polish side is said to have stated that no videos were available and that the recordings had been deleted early on, as required by law. [...]

(https://www.tagesschau.de/investigativ/ndr-wdr/nordstream-172.html)

So even if they are actually not to be blamed for not preventing the border crossing, they did nothing for 2 1/2 months... to then report back that the suspect isn't in their country anymore.

Just like they refused to hand over video footage all this time to then make a statement recently (so half a year later...) that it was deleted in the mean time.