I wouldn't call it propaganda or even news - it's just theories at this stage.
What we can speculate about is motive to deceive. Russia has been incurring some notable losses from Ukrainian anti-air defences recently, so there would be a motive from the Russian side to portray those anti-air defences as either ineffective or untrustworthy so as to try and sway public opinion about its use.

Claiming that POWs were onboard the plane aligns with that motive but it also raises questions such as:

1. The plane was reportedly shot down after taking off from Belgorod, so if it was carrying POWs away from Belgorod, what was the intended destination? It doesn't seem logical that Russia would fly from Belgorod into Ukraine (unless they were stupid or taking the risk).
2. Why not transport POWs to Ukraine by road or rail, given that Kharkiv is only a 90 min drive away?