NBD cube nuroad c62 ex PROBLEM (maybe)

Hi everyone,
I’ve bought a Nuroad c62 ex di 1*12 Shimano grx di2. I’ve noticed a specific grinding/friction noise when backpedaling, especially when the chain is on the larger cogs (steepest climbing gears).
The shifting is indexed correctly and works fine when pedaling forward. Is this level of noise/resistance normal for a 1×12 setup due to the extreme chain line, or should I be looking for an issue with the PressFit BB or derailleur tension?
Thanks for the help.