I have also just got this blind motor and set it up today, mine is branded Kogan SmartHome (Australia) but is definitely the same model, via Tuya, just rebranded for Australia.
I’m also having troubles with setting the position (e.g. open to 90%).
Tuya app: I can see and set the position to any percent
Google Assistant: I can say “open blinds to 50%” and it does it.
Home Assistant: I cannot see the position and I can’t use the cover.set_cover_position service (i.e. cannot set it to 50% in home assistant using cover.set_cover_position service).
HA + Google: I can use the Google Assistant Relay component in HA to send command to google to se the blind to x %. e.g. “open blinds to 90%” and that works! What I think I’ll do is set up an input_number slider and have the number sent to google when it’s changed.