There is the denial factor, but IMO it would also be how, why and who became prisoners to begin with that would stick in minds of the Poles.
Quote
The victims were Polish officers, soldiers, and civilians captured by the Red Army after it invaded eastern Poland in September 1939.
Strictly speaking, even the Polish servicemen were not POWs.
The USSR had NOT declared war, and the Polish commander in chief had ordered his troops NOT to engage Soviet forces.
But there was little the Poles could do.
On 28 September, the USSR and Nazi Germany, allied since August, partitioned and then dissolved the Polish state.
They then began implementing parallel policies of suppressing all resistance and destroying the Polish elite in their respective areas.
The NKVD and the Gestapo coordinated their actions on many issues, including prisoner exchanges.
At Brest Litovsk, Soviet and German commanders held a joint victory parade before German forces withdrew westward behind a new demarcation line.
The Soviets helped the Nazis defeat Poland and were allied with Nazi Germany.

They had the gaul to lay the blame on their former ally Germany for the massacre after the germans discovered the Soviet's handy work in Katyn forest and exposed the massacre to the world.
The Soviets did manage to convince Britain and the US it was innocent and it was the germans who actually murdered them.
It was a lie that lasted more than 50 years, until hard conclusive evidence proving Soviet guilt forced some admission of guilt from them and eventually admit the full truth.
Quote
Some of those who died at Katyn included:
1 Admiral
2 Generals
24 Colonels
79 Lieutenant Colonels
258 Majors
654 Captains
17 Naval captains
3,420 NCOs
7 Chaplains
3 Landowners
43 Officials
131 Refugees
And a Prince
Also among the dead were:
20 University Professors
300 Physicians
several hundred Lawyers, Engineers, and Teachers
and more than 100 Writers and Journalists
as well as about 200 Pilots.
It was their social status that landed them in front of NKVD execution squads.
Most of the victims were reservists who had been mobilized when Germany invaded.
In all, the NKVD eliminated almost half the Polish officer corps. -- Part of Stalin's long-range effort to prevent the resurgence of an independent Poland.
Poland is a proud, independant nation and its people bravely fought for their country's freedom in WWII and this was nothing more than a murderous plot by Stalin to help crush any chance of an independent Poland rising so it would remain under Soviet control.
It did remain under Soviet control for a long time as we all know.
(Just another reason for Poles to resent the Soviets.)
Cheers,
Dave