I guess you're right that sky background normalizes by the max value.
Quick HFR shouldn't make a difference in seeing the values (let me know if it does). What it does is just compute HFR on the center 25% of the image. That speeds up the computation considerably, as in some cases the hfr can take quite a while to compute on a raspberry pi. If you have auto compute hfr on, but yet hfr is not being computed, please send me a log with debugging turned on for at least the EKOS sections: Indi, FITS and Capture, and Verbose & File checked at the top of the logging dialog.
BTW, the value might read 0 if you have the cursor set to somewhere outside of a capture interval. Click on a capture in the timeline (one that's after you turned on auto-compute HFR, of course) and check the value displayed as well the value printed in the "Details" box in the lower left, and let me know if it's still 0.