I was at the vehicle limit in my save and kept trying to figure out why my vehicles weren't being replaced like I scheduled them to be even though they were 100% manufactured. I tried sending them to the garage and that just cancelled the pending upgrade. Then I thought to try to make one vehicle slot free. Turns out that was the fix.
As soon as I freed a vehicle slot the upgrades started working normally.
Confirmed in v0.7.3 w/ 170 vehicle limit and t2 vehicle garage being supplied automatically.