Top Floor Toilet Recurrently Blocking

I have a toilet that blocks up on the top (2nd) floor of a property.

The toilets on the 1st and ground floor are OK)

Blockage clears when the using a plunger. But returns within a few days. It seems like there is something in the pipeline that is catching toilet paper. Once enough toilet paper catches it blocks. Then plunging can clear it again temporarily.

I put a USB camera down the toilet. there is nothing at the neck of join where the toilet meets the floor plumbing.

I tied the USB camera to the end of a toilet snake/auger/drain rod and was able to push the camera further down the toilet. The pipe does a horizontal run after entering the floor. This length of pipe is full or semi-full of water even after plunging has cleared the blockage and flushing works again.

I don't think top floor horizontal plumbing should be full of water? That would maybe suggest that there is something stuck at the end of the horizontal run that is damming and then recurrently blocking the pipe once enough toilet paper aggregates.

A plumber has suggested that the next step would be to physically remove the toilet from the floor and use their CCTV to investigate.

Any other ideas on how to investigate/resolve?