I have now twice, in different saved games, experienced the following: value "3" minefield counters from random draw behave differently from others. They show their masked value to all player roles (Axis, Allied, observer, Solitaire). This happens both in the map and in off-board holding box.
I am using Minefields extension from Vassal Academy. The other extensions I have active are from Cassino module.
The image below is from off-board holding box viewed as Axis player. The leftmost "3" is unmasked on purpose.
This image shows the counters as viewed in any other role. I have closed and re-opened the OBHB after changing roles.
However, when I draw "3"s from Game Pieces -> Minefields they behave as expected (*). "3" have been replaced, OBHB viewed as Axis:
OBHB viewed in any other role:
*) When moving Game Pieces -> Minefields counters of any value, the mask toggles on/off with every move. I have seen this both on the map and in OBHB.