I tried in my own game and I see you are correct and can't do any damage to Belize fort at all. Someone else also said that Martinique fort will suffer damage only to about halfway, then no more damage. I had made a change to tracing efficiency, and had tested it on a couple forts with no problem, but now I see that there is some sort of difference between certain forts.
I sat down to troubleshoot and find that it is indeed only certain forts and was due to that new tracing efficiency. Found the issue, corrected, and as soon as it finishes compiling, I will update Itch. The problem was the engine, so just Maelstrom needed an update for this fix. It is now uploaded, so you should find that you can now hit the Belize fort properly.