I'm having the same problem. I contacted Paradox support and they sent me here.
As you were told in your ticket, you are not missing the Mass Transit maps as such - you are missing something else the particular map you are looking at needs.
I've now had time to do some more digging on this. According to this post
the game and steam says I have mass transit in the library however when I try to start some maps it says mass transit missing and wants me to buy it again why is this?
steamcommunity.com
basically they are actually the Map Pack DLC maps mislabelled - the actual maps that come in MT will be there and okay.
The only maps that come in MT are:
- Arid Plains
- Regal Hills
- Seven Lakes
Do you have all those okay?
The map pack comes with:
- Biomes Valley
- Desert Pass
- Farmland Flats
- Meandering River
- Mountain Taper
- River Valley Plain
- The Eyeland
- Tropical Pass
They will not be available until you buy that DLC.