What I am about to describe has not been reproduced but since I believe it was introduced by some sort of deployment by Microsoft, it's a bit out of my hands.

In May 2018, I had created a number of pages using the Modern Page template since I was embedding Power BI reports into SharePoint pages and this is the only template that allows this. The last time I edited these pages was June 4 2018 at which time all the pages had the title header image removed. Between then and June 25th, ALL of my modern pages had the "silly' Title background image re-introduced (as if someone had "reset the title image" setting).

I am going on a hunch this was related to some Microsoft deployment change. If true, please DO NOT reset default property values on existing Modern Pages.