Actully NAPA fighters coming from east of FOB Manhattan are not attacking, they run, but they are there just inspect and see who we are. When they hear who you are, depending on NAPA weapon cache was reported or not, in Arma2 (but apparently not Arma2 CO) they should either offer help to escort or just give some tips and then say good bye. Even if reporting guns they can be followed on their long journey to NAPA safehouse where they stay until the end of the mission (and sometimes get killed there or on the way there by enemy patrols).
Jezuro told me he could not reproduce the issue in CO. It is still unclear if it is because using some developers god-mode to clear Manhattan mission enemy main camp and 4 sentry camps which are required for mission completion. Apparently he did not (yet) use my 1.59 multiplayer savegame which includes Manhattan with all objectives completed and it is located right next to Gorka church so you can make either decision to report or not. The fact that it is MP savegame should not make any difference since xALIENAx reported earlier in this thread that he has only tested singleplayer and it happened to him there too.
So my best guess is that bug only happens if weapon cache objective is completed as one of the last ones in Manhattan (like I have done twice, I left is last one before Report to Cpt. Shaftoe (for second time).
My second best guess could be that the way how we play Bitter Chill start before FOB bombing changes something that bugs weapon cache decision.
In Bitter Chill start I have always avoided russian contact in Spykayev's house (I go there before 7-10mins of mission starts and russians arrive to house only after 10mins) also I go around Chedaki patrol in FOB Manhattan frontpost (road leading down from FOB Manhattan). Instead I climb to FOB Manhattan on northeast part of the hill (without visual to Chedaki). So this could result me beeing to fast in returning to FOB Manhattan.
So what we should test next would be to play in SP (so it is easier for Jezuro to use that savegame) and go to weapon cache as the first thing and then do only main camp and 4 sentry points.
I'm afraid I have not time to do all that again, but if I do in following weeks, I'll report here what happened.
Meanwhile if you have anything new to tell, let us know. Thanks!