The way I fixed it was by switching the vehicle that kept bugging out. So let’s say I want to dupe the Issi but it keeps disappearing.

Step 1: Go outside your bunker and call your mechanic to send you over another car. Step 2: Drive that car into the bunker and put it in the MOC. (That should take out your Issi from the MOC and put it in one of your garages.) Step 3: Leave your bunker and do the same steps. (Call your mechanic and drive your Issi that kept disappearing into the MOC. This should send that other car back to your garage.) Step 4: Now just follow the normal duping steps and your Issi should appear in your bunker as normal.

Hope this helps! After fixing it I’ve been able to dupe around 20 vehicles and whenever it doesn’t work I use these same steps. GL!