BF10 and other patrol grids....

So why is BF the only grid that this happens with?

That depend upon who scripted the messages.

What makes sense is what happened - they screwed up.

Not necessarily. If you examine the reproduction KM gridded chart that came with AOTD (a far nicer aid than that which came with SH3) you see the ALL medium sized square groupings sharing the same first number (BF-11 through BF-19 and EJ-41 through EJ-49 for example) have their borders thicker compared to the numbered squares but thinner than the lettered squares. Why would the actual originators of the chart do that if not to highlight those nine squares to be use with a single designator?

Try finding BF31. Bdu ordered me to patrol it

English Channel including Southampton, the Isle of Wight and the Solant? Easy pickings there in any U-Boat any time; if by easy you mean suicide by escort...