Designated Hitter Penalty?
 

I was using simulation module for projections and trying to find the best lineup, but encountered very weird thing.

I have two very good power hitting Right Fielders, but whenever I put one of them at DH they suffer the penalty to their OPS, about ~0.025 (all around drop in hits, walks, HRs, increase of K's).
When I put them both in fielding position (slotting one of them at LF) they are back to their normal hitting.

The sample size is decent enough with 20k games.

Stamina is also not a factor since I use rest day after every 5 days to simulate the environment of my league and even if they should tire less at DH so any effect of it should've been reversed.

Is there some build in OOTP penalty to hitting as DH or am I missing something?

Late reply.... but

some research here:

https://mglbaseball.com/2013/12/09/p...ties-revisited

which suggests that a player becomes 5% less effective at the plate when they are a DH in real life, than when they are out in the field.

So you are very observant, and it is likely that the game engine includes this, given Markus's affinity for SABRmetrics.