Steam Unlocking The Wrong Achievements On Compatible Content

Since some compatible content is missing from my library right now due to the Steam issues I decided to finally give some content that I got from the recent TSW3 Humble Bundle a try in TSW4. I decided to drive the BR Class 20 on West Somerset Railway. I guess I should have done the tutorial first, as I wasn't able to get the loco moving at all... but after I quit the game later, I discovered that Steam now says I have earned an achievement for driving a 200 Miles in the BR Class 20!



Just to stress this again, I have never touched the Class 20 in any previous TSW game (but I did own it in TSW3) and the one and only time I used it in TSW4 so far, I completely failed to get it moving. As you can see in the picture, the progress bar showing miles driven is actually empty, as it should be, but I still got the achievement. Something's definitely up with the way achievements unlock on compatible content.