I can confirm that this has happened to me as well, following the 1.3.1 hotfix that was recently issued (which was supposed to fix another issue with Miller Peshek).
According to my map (screenshot: https://imgur.com/a/zefSc), he is now located to the far bottom right corner of the playable area. I even took the long hike deep inside the Rattay woods in order to find him, but I’m hitting an invisible wall just before I reach him (out of bounds, Henry is saying that he cannot leave).
This issue has been reported in the other following forum post: Müller Peschek verschwunden
So are there any console commands to respawn Peshek to his proper location, or do I need to wait for another patch or hotfix?