Yep I found this out the hard way. During the wargames mode if you replace a trap you shouldn't have to use materials from your own back pack either because it won't be there upon returning. I replaced three wall launchers and some gas traps using my own materials and when I returned the traps were also gone, not sure how this is going to work out in the long run.
Hey there, this is intended and was covered in the v9.0 Patch Notes.
" When you start a Storm Shield Defense or Wargames mission, your fort’s layout is saved. After the current Storm Shield mission concludes, your fort is restored to the saved state. This includes all structures, and traps (durability) placed before you start the mission. Any structures or traps that are placed after the mission starts will be cleared after the mission ends."
However, the bug the OP mentioned of the durability not resetting properly is an issue that we're looking into.