When changing any of the "Quick Render" settings, the preview jumps from what you currently see back to the basic preview and then after a few seconds to whatever change you made.

0- Let's call the preview that you see by default the "basic preview"
1- Click the Preview button , after a few seconds (depending on pano size) the preview updates. Let's call this the "Quick Render (QR) preview".
2- Change anything, e.g. turn on the Exposure checkbox. The QR preview changes back to the basic preview
3- After a few seconds it jumps again to your modified QR preview.

What should happen is that you keep seeing the preview from step 1 right up until the new preview is ready to be displayed in step 3. It should not drop back to the basic preview in step 2 (this is the same one you see in step 0).

See the screenshots below if it's not clear.

This is basically the same problem as the switching projections bug that you already fixed. It would improve usability if you fixed this too.