Litter Box Nightmare... STOP POOPING ON THE FLOOR.

I've thoroughly scoured this Reddit but nothing seems the exact same as my current issue...

I have 2 indoor cats, male and female, neutered from kittens, brother and sister, 8 years old.

Brother cat is the issue.

I'm going crazy. He has been to the vet, checkup complete, full blood work, £300. Nothing wrong.

He has had issues with cystitis before but no current symptoms (blood in urine, frequent box visits, yowling while using the box)

In fact, no urine issues at all, it's just poop.

he keeps pooping on the floor next to the litter box.

I've currently tried:

-Thorough cleaning of the floor with enzymatic cleaner in case of any residual smell.

-New litter boxes with lower sides.

-Extra box. (from 2 to 3)

- Cleaning every morning and night, and any time I pass the boxes to use the toilet.

-Moving the boxes.

I've used the same litter for years, soft clumping wood based... last time I attempted a switch they would only use the box with old litter so I don't know if that's just a waste of time.

Night time seems to be the main issue but I have witnessed him do it in the daytime too (I hear him scratch the floor and go to check)

I've found poop on the floor when all 3 boxes are empty. I caught him peeing in the box, getting out then just taking a dump on the floor.

I'm totally baffled? is this my life now?