Warning: the light meter app on my phone ruined 2 rolls at Joshua Tree

I went out to Joshua Tree two weeks ago to shoot the rock piles at golden hour with my Pentax K1000 and a roll of Portra 400. I left the battery for my in camera meter at home like an idiot, so I pulled up a free light meter app on my phone and trusted it. Shot 36 frames of what looked like perfectly exposed desert and got back 2 rolls that were all over the place, some blown out white, some so dark I can barely see the rocks. The thing is, phone screens and phone cameras read light in a way that does not match how film sees it, and I knew that in the back of my head, but I told myself it was fine. I met a guy out there with a Sekonic spot meter and he let me compare readings, and the app was off by almost 2 stops in the shade. So now I carry a real meter in my bag every single time, no exceptions, even if it weighs a ton. Anyone else get burned by a phone app and just go back to carrying a real meter?