Addendum:
I found an issue at github "HelloVolla"
https://github.com/HelloVolla/ubuntu-touch-beta-tests/issues/129
Please also comment or vote for it.
I am asking me, if it is a hardware or software problem?
How can we narrow the problem to help the dev to solve this issue?
Unfortunately I am not a dev, but have no problem to import a fix and play with it to get it fixed.
Best regards,
zintor