AI teams decide on their own to board vehicles when said vehicles are in user defined layers (as opposed to default "Empty" folder).
On game start vehicles will be assigned automatically to the AI team, trying to unassign the vehicles via script will result in the vehicles being re-assigned again automatically a split second later.
If the vehicles are taken out of the user defined layer and moved to root the AI will behave as intended.
If the length of the patrol route is shortened the AI will proceed on foot, however the assigned vehicle remains.
If all the waypoints are removed, the AI wont have vehicles assigned anymore.

Could be reproduced on both diag and normal exe on dev branch, and on normal exe in stable branch.
Confirmed by @Lexx2k as well.

Repro mission:
{F2827785}