There is only two parts involved with the backlight: the logic board which is has the MOSFETs & inductors which controls the three backlight strings and the display assembly its self which has the backlight LED’s.
Your symptoms implies the LED driver logic is messed up on the logic board. If you want to try to fix the driver logic you’ll need to grab the schematics - board: 820-2997 drawing: 051-8774
Or just replace the logic board: [https://www.thebookyard.com/product.php?cPath=92_93_293_473&products_id=9043|27" Thunderbolt Display Logic Board, Apple P/N 661-6060]