When any mob walks by any vehicle, the mob will be sent about 2 blocks underground, or at least until it's head is submerged. However, it won't always suffocate.

Is the vehicle moving during this time, or is it staying still? Because I've always had issues with moving vehicle moving players in places they don't want to go due to MC trying to keep the player out of the vehicle's collision boxes.

engine off, completely still.

Very odd. I haven't seen that before. Do you have a video you can post?
EDIT: Which version is this on?

I can provide a video, but it will likely be tomorrow. It is on the newest version.

here is the video: https://youtu.be/tZELEVDt6xY

Odd. I've never seen that in my tests. And I've run into plenty of zombies. Would you be able to join the Discord so I can throw some test builds at you? It's easier for me than doing things through Git. Just ping me when you get on. Username is the same.
Link: https://discord.gg/KaaSUjm

I joined, thank you

I'm pretty sure this is fixed in V19.5.0. If not, please let me know or re-open the issue.