If you first read back into a native array, then do a normal readback, the normal readback can leak memory, if it reuses the request that was used by the native array.

Reproduction steps:
1. Create or open any unity Project
2. Add the attached C# script to the project
3. Add it to an object in a scene with just a camera
4. Enter Play Mode
5.a Monitor Memory usage in the Task manager or
5.b open Window/Analysis/Profiler and add the Memory Module

Expected: Memory usage rises up to a point and stabilizes there
Actual: Memory usage keeps rising

Reproduces in 2019.4.2f1, 2020.1.0b12, 2020.2.0a16
Does not Reproduce in 2019.2.0f1