By refusing to invite Russian President Vladimir Putin to the WWII commemoration ceremony Poland attempts to turn Belarus and Russia against each other. They hope that if [Belarusian president] Lukashenka comes to the ceremony and Putin does not, there will be bad blood between two of them.
Poland's decision not to invite the Russian delegation to WWII commemoration ceremony has to do with Russian aggression against Ukraine. Krzysztof Szczerski, chief advisor to the Polish president, stated in March 2019 that the anniversary ceremony will be held “in the company of countries with whom Poland now cooperates closely for peace based on the respect for international law, for the sovereignty of nations and of their territories”. This point was reiterated by Jacek Sasin, Polish deputy prime minister in July, who said: "I think it would be inappropriate to mark the anniversary of the beginning of the armed aggression against Poland with the participation of a leader who today treats his neighbours using the same methods."
See earlier disinformation cases alleging that Poland attempts to destroy the world order by not inviting Russia to commemorate the start of WWII and that the real reason behind the refusal to invite Russia is the Russian ban on the imports of Polish apples.