[2023] Harbour buildings cannot be built due to lacking free space despite enough space/resources being present

Hello,
I've looked if others had this bug and I found some discussions, but those are years old at this point. I'm playing on an old campaign save, all "big" (i.e. with new functionality) DLCs are active. On my main island, I recently destroyed some quartz buildings on my coast because I didn't need them anymore, but the resulting free space doesn't seem to be registered as free by the game. I cannot build there, no matter how small (not even a single decorative tile), they all error out with "Free harbour area needed" despite there being plenty of free space available.


As requested in the how to post a bug thread, here's my save where the bug is present. It happens on the main campaign island (Ditchwater is the original name I think): https://mega.nz/file/lEsXlSRJ#nEKKvaQX0yjIaSX_bqFopFz33jBywfhUWqRaEFtVmWw: (https://mega.nz/file/lEsXlSRJ#nEKKvaQX0yjIaSX_bqFopFz33jBywfhUWqRaEFtVmWw)

I have no idea what is causing this. Am I doing something wrong? Thank you for any help.