I'm posting this as suggested by Power BI so that the issue, and my resolution can be investigated to benefit other android users.
This is an issue I had in the past when I tried to log into the Android power bi app. I kept getting the message "Could not authenticate because your corporate SSL certificate is untrusted by this device".
Initially, I had no authentication issues with my Samsung Galaxy Note 8 (and 4 prior to that).
I'm guessing that either Samsung or Google have made changes with installed certificates as this issue didn't occur on Apple iPhones (corporate or personal)
After multiple searches, I found this link that reviews certificates used by a site and creates a report for you.
I typed in the domain of my organisation and got a report detailing what certificates it uses. The report takes approximately 2 minutes to complete.
I looked for the Issuer for each certificate and went to the link provided for that certificate. I then downloaded and installed the certificate. I also did this for app.powerbi.com and installed a missing certificate. I'm not sure if you have to reboot your device but I did.
My Power BI app now works. I'd like to add that in addition to Power BI not authenticating, I could not log in to any office apps or Skype for business.