Solved

Inexplicable retirements

Fully repaired as in engine changed as well, right?

That’s a weird one if you repair, replace engines and still retire. ?

By the looks of it, you were doing 12 laps stints on both cars when they retired, and if your cars were repaired (Parts), then your drivers costed you the race.

A common, although annoying, problem in offline races and some live ones is when the driver tries to do an extra lap if their remaining fuel is ≥80% of a full lap. It can cost you a lot of time, mess up with the strategy and even retire the car if your pit box is too far down the pit-lane.

For example, let's say you were using 2.81L per lap and had to go 12 laps (33.72L at PL3), but fueled in 36L for safety. By doing this, you triggered the 80% 'rule' (2.248L remaining) and your driver would do another lap, even if it means ruining your race.
It was a common thing on the re-fueling era of F1, so you aren't fully to blame on this one, but the smallest of margins are the difference between a perfect stint (≤0.1L) or retire (≥80%), especially in shorts tracks like Singapore and Brazil, and/or those with long pit-lanes like Austria and Spain.

In Offline you need to be even more precise than live racing, so learn from it and nail the next one. Good luck ?

I think the race was the last one in Australia:

Race ID: 51992739
Cars in 28th and 29th.