I was surprised to see Google when I opened up my browser for the first time today. Sometimes it opens a new tab automatically, but it’s never done anything like this before. I took a screenshot and that of a normal tab for comparison.
I didn’t even know Startpage had a mobile browser. Just because they’re good at search doesn’t mean they’re good at building a browser.
If it were me, I would check what settings it imported from somewhere and see if it’s somehow set Google as the default search engine. It might have picked that up from a never-used default bloatware browser you can’t uninstall (i.e. Samsung Internet, etc.). Though I agree that the usual cast of Waterfox, Ironfox, Brave, Chromite and setting Startpage as the default search engine is the way to go if you actually want privacy features.