Hello,
For a long time, and almost since the very beginning of my FP5, I got the issue that I had invisible text in some of the preinstalled android apps. Only on these apps, and never on independent apps. For example Agenda, Sheets, the alarm app, Maps etc. The text was there, but it was invisible to me, as if the color of the text was transparent. For example in the menus, I could only see the buttons, and not the explanations or categories. Everything was empty. After a lot of trials, asking dev people friends, searching the internet and this forum, I was totally blocked for a long time. I just tried again with an IA and it found the solution, I must say it’s quite efficient to solve PC issues and so phones as well it seems 
The solution in my case was to make the update of the following apps:
- Android System WebView
- Google Chrome
- Services Google Play
And also to empty the saving “cache” of all the apps that were giving an issue.
Then it was finally solved!
So I post this message just in case someone has the same issue, if it can help. If a moderator can help put this ticket in the good conversation, with the correct tags etc. so that it’s more usable, please do it, I almost never go here so Im’ not very familiar to the way the forum works. Thanks in advance!