Remote scheduling failing - "!Error, cannot find event to schedule"

[prpr]

It occurs to me that if really was retaining all the EPG data since April last year,

It's not all the EPG data, just a very few items.

then it would have contained two 11th of Mays. Could my Humax have been looking at the first one, from 2025, which wouldn't have contained a match?

No. For one thing the timestamps are handled as Modified-Julian-Dates as transmitted, not year-month-day.
I haven't looked at the file format of the epg.dat file for ages, but I expect it'll either be the same or Unix epoch time style. The epg.db certainly contains the latter.
But the search as detailed above only checks the service-id and event-id anyway to find a match, so it's irrelevant (time sanity checking comes later). It's puzzling, but we don't have the data to investigate any more.