I see that a lot of player experiencing this.
PTR 1.35.0.19805 Bug - Campaign instant defeat - PTR Discussion and Bug Report - Warcraft III: Reforged Forums (blizzard.com)
I do not know what can cause it for some and not for others. Hope it will be solved until the patch go to live.
And me and others also reported it at some thread, but no reply from blizzard. As usual. 