WS2812b flickering at low levels
The most obvious issue is that you have connected together a device with 3.3V data output and a device that expects 5V data input. They are not compatible without a level shifter in between.
When you lowered the 5V supply down, it also lowers down the data input voltage requirement so that is why it works better with a 3.3V output.
The WS2812b datasheet says the Vih is 0.7 times the supply voltage. The fact that it has worked for you without a level shifter before is pure luck due to manufacturing tolerances and differences between batches of components. It may or may not work when used outside the specifications, while operation is guaranteed only when used within specifications.