Microshift Advent Super Short Skipping in 9th

I've just received a Le Petit Porteur eBike ready kit. I installed the TSDZ2B (250w, no crazy power) motor (with the 2 included shims on the drive side), threaded the shift cable through the bowden and secured it to the derailleur. I followed Microsift's chain sizing instructions but I'm getting some skipping in 9th when under heavy load. The derailleur adjustment looks okay, but neither my colleague nor I (bike coop) have any experience with these super short derailleurs.

I slacked the high limit a little, just incase it was climbing into 8th, but that didn't help.

Looking at the chain length in 1st, I don't think I can go any shorter.

The photo of the chain length in 9th looks slacker than in real life.

Any suggestions? Thank you 🙂