@addictedgamer02 - Yeah, using the ENT and selecting the 'Disable Despawn of DLC Vehicles' in the Vehicle Options menu, I was able to spawn and keep the new cars as normal (none of the in-game shops worked though). Without that option checked, they disappear just as they've always done.
@Noir1998 - people in this thread are saying Menyoo doesn't work (to spawn the latest Arena Wars cars)
https://gtaforums.com/topic/788343-script-hook-v/?page=228&tab=comments#comment-1070583801
@Anonymoused281 - what trainer are you using?
@addictedgamer02 - Yeah, using the ENT and selecting the 'Disable Despawn of DLC Vehicles' in the Vehicle Options menu, I was able to spawn and keep the new cars as normal (none of the in-game shops worked though). Without that option checked, they disappear just as they've always done.