FTB Chunks lets players claim chunks, which prevents other players and destructive mobs (e.g. creepers) from manipulating blocks in those chunks.

It also prevents Create's mechanical plough from tilling dirt or grass into farmland. The plough can still break blocks, so it seems like this is specific to the tilling action.

You need to ally the plough fake player (yellow at the bottom of the allies list)

I am pretty sure this is intended behavior, so that you can't 'edit' someones chunk using fake players if they don't have them allied. If anything it might be worth asking FTBChunks to auto-ally fake players or have a config option for it.

Can confirm @19175's workaround works. Thanks 👍
I'll keep the issue open and leave it up to the maintainers to decide if this is something they'd want to address.