Upon reviewing things, I think the issue is in U3 (the FET Driver). The IRS21850 has an undervoltage cut-out of nominally 8.2v - but can be as high as 8.9v per the spec sheet. Combined with other worst-case voltage drops of D16 and Q1, we think this is the issue.
A good corrective action will be to replace U3 with a different driver chip. The IRS21867 is a dual driver chip (High and low side), but its high-side is pin compatible with the existing PCB layout. Plus, its under voltage cutout is 4.5v - much more in line with the FETs needs. So, a proposed errata for v0.1.4 of the PCBs is:
- Replace U3 with IRS21867 driver ship (Mouser URL: IRS2186)
- Solder bridge U3 pins 3 & 4 (this will pull the unused low-side drivers input to a fixed state)
I have updated the Mouser BOM (link here) to include BOTH the old and the new driver chips. When ordering you can select which one you wish to use. Once I am able to verify the function with the IRS2186, I will remove the original IRS21850 option.