As far as just the issue with flickering goes, I had the same problem with a U14, The repro MPU board I had (made by Two bits) was fitted with a 4049B at U14 from the factory, causing flickering, would have been flickering from day 1.

I read another post a while back from barakandl (thanks) that stated is must be a 4049UB not a 4049B. Changing it fixed it.

I stocked up on some 4049UBs from here, they were genuine parts that I received & not remarks/fakes.

https://www.taydaelectronics.com/cd4049ube-cd4049-4049-ic-hex-buffer-converters.html

I hope it's the UB version that got soldered in for good?