Tach output changes when switching from BLDC --> FOC

We recently switched our fleet of robots from running in BLDC mode to FOC mode. In doing so, we noticed that the reported distance from each motor (which reads from the tach value) changed dramatically. In BLDC mode, we noticed a +- 5% difference between any given robot's reported distance (for the same actual distance traveled). once switching to FOC mode, the motors started reporting up to 200% the actual distance traveled. Even stranger is that by changing the FOC detection parameters (detection at 50% duty cycle instead of 30%), I can get the motors to report near 115%, but never lower.

My confusion comes from the fact that I assumed that the tach values came straight from hall sensors and had nothing to do with the motor control mode. Does anyone have an explanation for this and/or some ideas on how to get more consistent results? The distance reported is extremely important since it is the foundation for our localization system.

Thanks!