I (UK user) have been having the same issue when clicking on attachments that used to open in a new tab in the browser. Now they just download so you have to open them manually. Doesn't seem much of an issue when written down but in practice slows down entering expenses and bills by a lot and makes QBO frustrating to use. Like yourself I have gone through using multiply platforms and browsers to find that it QBO that is the problem. 

QuickBooks Support on the issue would be laughable if it wasn't so condescending as just in this topic there has been multiple people complaining of the same issue only for support to tell us how to set up the browser or clear it's cache and history which has no affect on QBO problem. I use other online software that continues to perform the simple task of clicking on an attachment and opening it in a new browser tab again ruling out browser or platform issue. 

I hope they acknowledge the problem and fix whatever they had broken until then I will not be placing anymore clients on their platform.