It does not. There is no inherent property of Marxism that causes one to be more likely to publish "bogus propaganda". Indeed, we can demonstrate evidence of the exact opposite, with the bevy of bogus corporate-funded studies (the tobacco industry is particularly notorious for this, but is by no means the only one).

You are making a non-objective argument in bad faith to grind your axe.

The issue instead is that conclusively studying human behavior is hard, and effect size is often small, and it's inherently impossible to control for conflating factors.