I literally was just looking into this the other night. I reduced the number of AI but I don't know if I was working with less than 21 AI.
Anyway, when I would start the race session with a rolling start, I along with all the AI would start out in space below the track, free fall for a second then plop in the garage spaces and the AI wouldn't do anything. Looking into the AIW I noticed the alt grid positions were set out in space below the track. In the AIW I simply copied all the "grid" spaces using a text editor and pasted them over the alt grid positions. When I rolled over to a race session it worked everyone lined up on the front straight and rolled off with the pace car.
I was going to see if I could contact the original author to update the track with my re-packaged track. I can try to upload it and provide a link to my version for you this weekend.
When i redo the AI fastlines plus other issues for various tracks i simply make a small AI update mas file addon for my friends to download thus no need to download the whole track all over again. Also, this method doesn't interfere with the original track's mas file and allows them to go on servers which don't have the update thus no mismatches. Give the small update to the author, a win win for everyone!
After checking further into the AIW i determined it needs much more attention before a update release!
1) Left/Right pace lines
2) Blocking line
3) Proper track thumbnail-icons
4) Improve blending the Pit-In line from the Fast line
5) SlowWhenPushed and AIDraftStickiness edits should be added.
6) Original AI fastline used from previous sim thus the fastline won't be as optimized like a new one.
AIW determines where the vehicle grid, pit, pace car, and garage positions are placed on the track. It also stores paths for the AI to follow and tells the AI where the right, left, and fastest path around the track. It can be edited in RF2 devmode but the process is cumbersome. The AIW needs to be added to the mas file and re-packaged together into the track to work properly.
The AIW I posted still needs what Ace found missing listed above. I've been busy with a couple other projects at the moment to look more into it. I was hoping my AIW could be a start with the grid positions not working even for an AI race. If someone doesn't get to it before me I'll post a new version of Belle Isle with this AIW.