Solved

Practice lap with zero time, 100% tyre wear

What did you try to do?
A practice lap

What happened instead?
App did the usual progress bar, then came back with "..." in the lap time area. Navigated away and back to practice times and "..." was replaced with 0.00 fuel, 100% tyre wear, 0:00.000 time.

Did the problem happen while using wi-fi, mobile data, or both?
Wi-fi

Did the bug happen in the app, on PC, or both?
App

What is the model of your device?
Motorola something, don't think this'll matter

What is the operating system?
Android

What is the version number of the operating system?
Don't know, don't think this'll matter either

Can the bug be reproduced?
No, previous and successive laps were fine.

Additional comments:
I have a screenshot but don't see a way to attach it on mobile

I'm afraid I can't now, I can't see practice laps once the race is over. I only tried it in the app, refreshing isn't really an option there.

It happened again today:

https://imgur.com/f6DooQD - not sure how to embed images, doesn't seem to want to.

It shows the same data when I open it in the app, having run the practice lap on a browser. No amount of refreshing or cache clearing helps.

EDIT: Turns out, some amount of refreshing will help - the times eventually (a few minutes later) came through.