Mapper 30 is unable to go into 4-screen mirroring. There is a variation of the board that performs exactly as the standard Sealie UNROM-512 board, but allows 4-screen mirroring. However, setting the 4-screen bit in the header puts the game in single-screen mode. This could seemingly be solved by making it an entirely new mapper, but seems like that would be a bit redundant since the two perform almost exactly the same.
I've attached a ROM for testing that has a different title screen on all 4 nametables, which can be switched with the 'B' button. If the 4-screen mirroring doesn't work, the final nametable that was loaded is the one that shows up the whole time because of the 4-screen bit in the header making the program stay in single screen mode.
Was fixed later on Github:
https://github.com/TASVideos/fceux/commit/9b6ff8fe490cae56f6a3bc24e0ac19e47b5eaa41