Enemy minefield indicator that won't go away

I'm playing a game on PBW where I discovered, and swept a couple of enemy minefields which were marked with the normal blue square with the "M" in the bottom right corner. I know now these minefields are clear (they've been swept, and ships regularly move through them without exploding). But the minefiled indicator square won't go away, and my ships were rerouting themselves to avoid the so-called minefields; and as one of them is on a warp point, that gets a bit annoying.

I can change this behaviour by un-ticking the relevant box in the Empire options - but then of course I run the risk of accidentally routing ships through real minefields.

Is there any way of solving this?

Mark

P.S. The game was running 1.67 when the problem first appeared, and is now upgraded to 1.78.