This bug is being filed for tracking purposes. As this is a problem with a beta OS, I don't expect any work to be done regarding a fix until closer to release. I will test the issue each time a new version is released.

Issue

If auto brightness is enabled for the built-in display, then the brightness controls (both hardware and software) become inconsistent, with macOS intervening after brightness is set and resetting the brightness levels by a significant amount.

Workarounds

Either:

• Disable Auto Brightness (through Better Display menu or Displays in system settings) to use brightness controls in BetterDisplay, or
• Disable Hardware and Software brightness controls in BetterDisplay while Auto Brightness is enabled