from my test the rear left wheel would levitate and wouldn't touch ground without load.
I would believe it's wheel or one of the block stock on each other's collision model.
Another problem is the structure the rear left upper trailing arm attached to is wrongly attached to the lower arm.
This should have the problem solved

@GrandPrix Upon reading my original comments, I realized I forgot to mention some points of suspension design. I apologize in advance.

You mentioned your suspension is very bouncy, which means there is not enough damping. Add damping always helps.

Another issue I found that can cause issues (but not necessarily) is small hinges and rotators. Simpleplanes seems to dislike those small parts especially when calculating the damping and constraint between two parts connected via hinges. This will make your suspension surprisingly weak because those swing arms are flopping. To remedy this issue, one of the dimensions must be close to 1. I learn this the hard way when making suspension for Allundra F5 Evo.

Also, remember to adjust the weight of your car with overload.

@Ownedpilot no, the suspension is very bouncy.

I think your suspension design also played a role here. The rear is unreasonably stiff so when there is any body roll, the rear wheel will catch air and lose grip. Because the spring in the editor is not compressed, you need to leave room for the spring to compress when it spawns in. One good example of this measure is the Lancia 037 I built. The suspension is very floppy so I left a lot of space for it to compress. I also integrated anti-roll designs to make the car handle better. Your car may not need that amount of travel but some amount is necessary.

Another thing. You can use funky trees to simulate the caster angle through angle of slip feedback. Again, the Lancia I built has the exact algorithm I use. It may be hard to read so I suggest you paste it to a notebook to read it.

Select each tire, and click overload, go to resizable wheel tab, go all the way down to where it says traction, change the 1 to a 2, test, change it more or keep.
Also brake torque: Default is 50% change that to 100. (or higher)
If you want to deal with the 4 slip options, Here is homework