I have the 28020 LL that was Conventional and SS. I upgrade to TMCC with C07 R2LC and just coupler and it worked. Had the PCB1-09F.
I have the 28030 NYC that came TMCC/RS and it had PCB1-0B9. This had DCDR so I upgraded to Cruise M.
Lionel made a ton of these back in that period and they flooded the market. These engine with TMCC were being blown out at about $150 by the big dealers.
So maybe just the very early ones had a bad motherboard. BAD on Lionel for not getting them out of the stock system and updating the parts site with the corrected board; if in fact they all were made that way. G