😞 This is a long-standing bug I'm afraid, and on my list of things to address (I did a thread a few weeks back of what I want to add / improve on the site over the coming months).
Basically if a mine is very close to the edge of a map sheet it doesn't show as being near to a mine that's very close to the edge of another map sheet.
I'm part way through writing code to covert everything to lat and long at the time the data is added to the database to store alongside the grid-reference. This will fix this bug and allow all sorts of whizzier mapping 😉
I'm away for a couple of weeks in the not too distant, then once back am planning to spend one evening a week working on this sort of thing until everything is sorted.
In the mean time don't worry about it - your data is spot on - once I've fixed the bug everything will magically work correctly 😉
my orders are to sit here and watch the world go by