FBL2360 Feedback update rate vs HALL RPM

Hello, I'm using Hall Sensor for Closed Loop Feedback sensor, it works as expected but not in slow conditions:
I'm using a 15 pole pair motor, operating in closed loop speed
The lowest speed at it will run, when Command is < 28 is Hall Speed 28 RPM, and Feedback 1 will fluctuate between 0 and 28 or 29
When I command 33 and above it will settle, Feedback 1 will match Hall Speed RPM 1 and works as expected
These values might change a little when I change the PID values, however in low values Feedback 1 does not follow Hall RPM 1
In Open Loop mode I can get much slower speeds, in Hall RPM as low as 5 rpm being quite stable.
Why is it that those values differ?

It seems that the controller is running into a quantisation error. Your hall sensor does not have enough resolution to accurately measure the speed of the motor at lower speeds, causing your stability issues. This white paper goes into the issue in depth but you should refer to equation 2.

If you would like to use closed loop speed control at lower speeds, you will need to find a method of position feedback with a higher resolution. An encoder would work well here.

Edit: You might also want to try closed loop speed position mode. You might not run into quantisation error since you are measuring the position instead of the speed in that mode. I am currently out of the office, but I can test when I get back.