Pixel loading pictures uses data not wifi

My pixel xl suddenly used vastly more data than I was used to on my previous phone, about 1.5gb in 2 days. I traced it to the free picture backup facility by Google. It seems the default is to do this even on data rather than wifi.

I went to settings, data usage, cellular data usage, pictures and told it to only do it on wifi.

It seems this is done when phone is on charge, but I charge my phone in the car when on cellular.