Sadly our app is a memory hog. Since this is the first time we have heard of this and the first time I have heard of someone using an iPhone 12 mini, I did some research.
This article points out that others have problems with apps crashing due to memory problems.
I can have my developer review the logs and see if he can see the issue but crashes like this would have a high likelyhood of being memory releated.
We need to come up with a process for reducing the memory requirements but this a big expensive task and as phones get faster and better we would expect it to become less and less of a problem.
I wish I had a better solution. Doing multiple short workouts would be one solution but that is painfull. Do you have any other devices to test on?