I am having the same issue with a Samsung Galaxy 8 running android. Avenza version 3.10.2.
I have changed all the settings referenced in previous solutions and when I turn off the screen, background GPS processing still stops.
I have also made sure all android and avenza updates are current and enabled background processing anywhere else I could find. But, background processing is confirmed to not be actually working in battery settings as no minutes of background processing have been documented, even after multiple attempts for it to process in the background.