He’s asking about the iOS app. While it’s true that you can run the desktop app minimized, that’s not really relevant (though perhaps helpful to other readers). Notably, the desktop app cannot have this problem: a running program will continue to run, regardless. This is not the same as the app lifecycle for iOS and Android apps, which can be terminated if resources are low and the app is not frontmost.
A TrainerRoad software engineer really ought to be able to answer this question.
I would guess that it is fine. This guess is based solely on the fact that the Android app requests the “prevent phone from sleeping” permission, which usually indicates that a dev has thought about it.
Try it out on an easy ride and let us know. Try to avoid switching back to the TR app for the whole workout.