@jlr Glad you found the issue, but you are right, it is strange that it did not work just remapped.
One thing I will mention is that lr-mame2003 actually sees two simultaneous inputs. The first input is the one that was probably remapped in ES and RetroArch configurations so it was fine for FBA and others. However, this version of mame ALSO SEES a RAW keyboard input alongside the retroarch virtual gamepad remap. It is strange, I know, but I wonder if MAME was seeing your raw left/right with the remapped one and together, they were cancelling each other out.
In any case, better to have it wired properly anyway.