Complaint: Carry on bag not allowed in the overhead

I travelled with Citiliner earlier on 2 October 2024 and the staff insisted that I put my carry-on bag in the hold in a bus where there were empty seats and more than ample room in the overhead. I enquired as to when the rule had changed when I was told, "You're not allowed to keep that bag with you" yet I have flown and travelled by bus and the bag meets the travel size requirements for carry-on luggage. I have complained in writing and requested confirmation w.r.t. the dimensions so that I am prepared next time. I still await resolution from Greyhound.