I have the EXACT same issue, except it always happens with one PDF in particular on my iPad mini 5. I’ve only had it happen once with others. The same thing happens on my computer with this particular PDF as well. It always opens to page 1 of the PDF, unless I choose a specific section under the bookmarks (e.g., just selecting Chapter 2 wouldn’t work); however, this trick doesn’t work at all with the iOS app. It has nothing to do with signing in and out; I tried it and no change.