Hi,
i have the exact same problemm and currently no clue how to solve this. If i connect both, Touch Display and an HDMI display it boots correctly into LibreELEC. I did not have to time yet to do further troubleshooting. Did you succeed in setting this up / solving this issue?
The only differences to you is that i am running the official 7" Touch Display by element14.
Thanks in advance,
Flo
What really helped was this thread:
RPi4 Composite out not working
My Pi 4 with Touch Display 7" is now running nightly build of today. When LibreELEC provides an successor
to LibreELEC-RPi4.arm-9.1.501.img that fixes this bug, i will change back from the nightly build.
In the meantime let's see, what bugs i will run into.