The above suggestion doesn't always work, you could try but as you say, with a fresh installation it seems very unlikely. I still have this problem on and off. It is not about scale, which base map, settings to export to pdf or transparency of layers or anything like that; i have been producing pdf maps for over ten years with esri software and i know about these. I honestly don't know the reason and I suspect nobody at ESRI knows. It is freak happenings like this that frustrate the workflow. You tell ESRI about it but it's hard to pinpoint and repeat the problem so you rarely get an answer. (Works fine with us, can't repeat your problem... Then frustrated you try again, honestly exactly the same way, and this time it works Until the next time it again doesn't work...)
They (ESRI and ICT experts) always say that internet connection/speed is not of any effect but I really do suspect otherwise. I can't think of any other explanation as to why sometimes things work and at other times not with exactly the same settings/workflow and that it happens only with online data. Could be your own internet connection but also the connection with de data centers from which services are hosted. I'm not an ICT expert or anything mind you, but at the same time no one has ever given me a satisfying explanation as to why the data connection cannot be responsible. The saying is always that it 'should' not give any problems.
One hopefully helpful thing I can tell is to clean up your Pro projects. When you are just starting out on Pro trying to migrate and get your workflow sorted again (like us) it's very easy to clog up projects with views, maps, layouts and random tables. I don't think there is a clean up tool in Pro as there was for your geodatabases in ArcMAP, so you'll have to do it yourself as best you can. Pro can easily become sluggish with too many maps and views.