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OPT3101: Trouble reading a reasonable phase value using a custom board and code

BenF
Prodigy 10 points
Part Number: OPT3101

Hello TI support,

I have a custom PCB designed to measure two distances in the range between ~1m and ~6m.
For this, I use two SFH 4550 each pointing at a 12deg angle from the center, one to the left and one to the right. I then use one SFH203FA photodiode at the center to capture the reflected signal of each of the LEDs.

I have a custom firmware that does not use the provided SDK, but I followed all the setup steps similar to the EVM and the SDK. For testing purposes, I have a very basic configuration of the OPT3101:
no-HDR, fixed LED power, only one LED channel is selected, frequency calibration-same as in the EVM, tested with multiple numbers of sub frames and average frames from 1 to 128.

I do the internal cross-talk calibration each time and the returned values seem to be consistent from time to time: IPHASE: ~20000 and QPHASE: ~ 4000
I can't get the illumination cross-talk to be consistent though, every time I run it I seem to get different values (very different). I am waiting for the required time for filtering, I block the light to the diode, but even if let's say it's not fully blocked, it doesn't make sense to get different values when i'm not moving anything at all and just re-running the calibration.

My biggest problem is this:
When the device is running, i'm waiting for the DataRdy signal and then reading the phase value from register 8. It doesn't act as expected, for TX0 the value is around ~7000 (but it changes depending on the LED current I choose) when pointing to a distant point, but as I move a piece of paper closer and closer to the sensor, the value gets larger and larger, and just before I get to saturation, phase gets as high as ~12000.

For TX1, the direction of the phase change is correct and it gets smaller as the object gets closer, but the values don't make much sense, distance-to-phase doesn't seem linear and when getting very close just before saturation, the phase value is ~12000.

The amplitude at register 9 does change reasonably for both TX0 and TX1, but I never see a value below ~1000 even for a very far object (many meters of no obstructions). All mentioned measurements are at a fixed LED current setting of 72.8mA.

Any ideas on what's going on? especially the negative phase-to-distance ratio with TX0?

Thanks,

Ben