Not specific about 515, but I noticed that after installing an unstable build, if I then launch the stable, and then click on "Manage Mods", this will bring up the mod manager still "hooked up" to the unstable directories. This is a problem when wanting to switch between unstable testind and stable playing. I know the first time it took me a few hours to figure out why all the mods had disappeared. Would be great if unstable modmgr used a different set of settings or registry keys or whatever it reads to remember the directories.
Don't you just have to choose the correct folders for Working Dir & Packages Dir in the bottom right of Mod Manager?
I checked mine and have the same issue. Two different installs from scratch (494 and 515), with 4 different folders (rFactor 2 Core, rFactor 2 Data, rFactor 2 Beta Core and rFactor 2 Beta Data) and did not copy anything from 494 to 515, nor even the controller.ini or setups. When I start the mod manager it shows the beta mods. Using the "working dir" and "packages dir" options en the UI works, but change the options in the other version too. Seems they use a common registry entry and not a ini file. Yes, it is a beta, but this behaviour can be confusing if someone have 2 or more rFactor stable installs (league, offline..) and they mix the settings.
I have been noticing this too. Sure it isn't a big deal if you are experienced but this needs to be sorted out. I have noticed for a long time that when you install a new build that it likes to set the Working folder to your Core folder rather than your DataPath where it belongs.
I think it's been like this forever, no? I'm used to it, but I know it could be annoying. I requested it use the data.path file.