I've seen something like that for sure, but it's been happening since release of La Resistance. I assumed it was because I had these a really long time, like 3+ years and maybe they just got captured somehow. Like I have been working one strategy where I infiltrate first thing in 1936 and sometime around '39/'40 the asset is simply gone. I recently did a test, without any reloads, where I just let the game run and the asset didn't disappear ever. I've looked for months for anyone mentioning this, but I've never seen mention (until you) that the asset disappeared. I suppose it makes sense that it's a bug.
My workaround up until now has been to simply prepare the mission and wait to complete it shortly before the war. You only need a 1% to initiate it, so it's not as bad as waiting till 50% strength.
