This looks like a problem that might be resolved by increasing the fidelity of the analysis. However, as you’ve noticed, there are some limitations on the level of detail that can be obtained in a large model with relatively small/thin parts.
My recommendation would be to examine the rear spoiler on its own. You can hide the rest of the car and create a new solution with a smaller fluid volume.
I actually hosted a webinar where I used a very similar example in a fluid analysis:
Webinar Recordings: Discovery Live Tips & Tricks
This article provides further explanation of fidelity and the accuracy of simulations in Discovery Live:
Discovery Verification and Benchmark Cases