I don't think it's meant to be a bug, I think sneaking commaders are discovered individually.
My original plan was to take out your PD with a smaller force while my mages are hidiing, while you break the siege next turn they will suddenly appear from nowhere to give you a nasty surprise. And if you didn't break siege I had six gate cleavers and a earth mage casting crumble so I could storm castle in one turn. But I didn't realise you'd move beforehand and patrol the province with your entire army
FYI, The army had 3 Air mage casting shimmering fields, 1 casting fog warrior, 1 will of fate, 8 for earthquake/rain of stones, 1 for army of lead, 1 for darkness, and 1 for the armor-destroying spell (forgot what's it called) and some other for antimagic, astral healing, transformation, warrior of neiflheim, etc etc. So unless you had fog warrior, your army shouldn't survive the 8 earthquakes, even then there were shimmering fields.
and now they are all gone, along with all their items and gems, oh fate how you mock me
