[Lisif Deoral]

It can't be that as all my minesweeper Groups can sweep more than 100 mines (useful if I run in two enemy minefields in the same sector, or lose a sweeper in combat).

I think Slick might be right, some of the minefields I cleared this turn had the tag automatically removed, while others were still marked. The minefields still tagged are on the more obvious places to place mines on, such as warp points and resupply depots - the kind of places where every player might want to place a mine or two...
BTW, it's a v1.49 game (I told you it was ooold!).