Just as an update to this. I span up a new VM with a fresh install of pfSense. Copied most of the settings across so it was ready to 'drop in'. I visited the site two weeks ago, shut down the broken pfSense, and booted the new one. Power cycled the cable modem and immediately everything worked. Two weeks in and the cable modem is still recognised and working correctly. So I assume the problem was caused by some sort of corruption in the config file.
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