You mentioned flickering which can indicate a cable problem. Cables tend to fall on a spectrum rather than being just "Good" and "Bad". So it's possible for a cable to work under one set of circumstances but not others. If the new firmware is negotiating the link differently it's possible a poor cable could start to show issues.
You can test another way. I would try setting display output to 60Hz and see if that makes any difference. If it still has the issue at 60hz then it's definitely not a signal quality issue since that requires much less bandwidth. If the problem goes away then I might try 120Hz and see what happens but I would definitely try another cable if the issue is dependent on refresh rate.
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