High Sierra update has completely disabled my Amphetamine.
All my settings no longer work on High Sierra. Example: Full screen brightness does not work, the screen light dims according to macbook settings.
You can try a SMC reset to see if that fixes the issue:
Also you can try deleting Amphetamine’s preferences:
• Quit Amphetamine
• Launch Terminal.app (/Applications//Utilities/Terminal.app)
• Type ‘defaults delete com.if.Amphetamine’ without quotes in a Terminal window and press return
• Re-launch Amphetamine
Hope this helps.
Same problem with me