Quectel UC25 in ECM mode + Fe1.1s chipset USB Hub = lockup/reboot on Pi Zero W

I’m having a weird interaction between components on my Pi Zero W setup.

I’m using a Pi Zero W with either a Waveshare USB Hub hat or a UUGEAR Zero4U to provide extra ports (I need 3) - OS is recent build of dietpi 6.34 with added dependencies.
Both hubs use the FE1.1s chipset.

When I run the EC25-EUX module in my sixfab 4G hat attached by USB everything is fine in QMI mode…online, data connection etc.

But, if I switch to ECM mode -“usbnet”,1 - then the setup either locks up or reboots - when the reboot starts it freezes at Network Connection start until I unplug the modem.

This only occurs if I’m using the FE1.1s chipset hubs - a coupe of old desktop hubs work perfectly in both modes (ECM works a treat in these cases) - so it’s a combination of EC25 & FE1.1s I think.

Power is not an issue I’m using a 2.5 A supply .

Anybody come across this before ?