Less brightness after plugging once my macbook pro to a thunderbolt display
You could try a NVRAM reset by starting your mac and holding the following keys: 

CMD + ALT + R + P.

(More info)

When your Mac restarts, you could release the keys.


  Some say that you have to do this 3 times for the best result, but I can't explain why


More information about a PRAM/NVRAM reset can be found on the Apple Support Website

When you use your Mac and connect it to displays or change the screen's brightness, it could be that your settings become a bit corrupted. You could erase those settings by preforming a PRAM reset as explained above. 

I can't find any known bugs on the Apple Diagnostics website, you could report it by sending a mail to feedback@apple.com