This appears to be an incompatibility with a recently released boot loader or eeprom for the 8gb version. Reverting back to the default 1.5.4 stellar mate os fixes the problem for the time being.
Yes, I have this too. On a 8gb pi 4, the 1.5.4 image works but running the update leads to no interface. I can still shh onto the pi, although this is not so helpful out in the field.
I've got the same problem here. Just bought Stellarmate OS 1.5.4 and installed on my Raspberry Pi 8GB with a 128GB SD card. I can get it to boot initially, but can't connect or get response after update.
I also tried booting from USB 3.0, but for some reason the wifi won't work when I boot from the USB, so I can't connect with the app, VNC, etc.