General tip with the minibike: Take the tires off when you walk away from it. When you do this the bike turns from a vechicle into a "block" i've never had anything happen to it once I took the tires off. A17 supposedly fixes the bikes falling thru the world etc, but i'll believe it when I see it. Iv'e played quite a few games like 7dtd and this is the first one i've played that has had so many issues with vechicles taking off on their own or just falling thru the world.
I have a theory though as to why a minibike goes thru the world, where it counts as a vechicle with it being fully assembled, it might load a split second before the ground, so it ends up falling thru it, because when it first spawns its spawning bascally in the air on top of nothing. Howeven when you take the tires off, the game treats it as a static block that cannot move and it has a defined position. This is just my theory on how it works, I have no programming experence. The fact that everytime I have had a minibike go poof when it was 100% complete, kinda makes me think this one, once I started taking the tires off, anytime I got off it, i've never had this problem again. I just take them off and put them in the basket.
I am more intersted in wether A17 will be able to keep up with the speed of vechicles, for example the engine just can't keep up the rendering speed on a high quality motor minibike. So it becomes a slide show as it has to keep pausing to load. I have my game on a SSD as well and it does this even on a fresh load of a world.