Feature not a bug?
I have been thinking about this.
I think they are both connected.
We know training camp results are rolled when collected, not when finished. But training camps have fixed results.
By forcing players to collect retraining to start a new one retraining, you allow retraining results to be rolled when finished. By forcing the collection instead of allowing a queue of 20 finished retrainings.
Forcing players to do one retraining at a time let’s the results change. The displayed results will almost always be accurate.
But if you let a player queue finished retrainings for 6 months, the displayed results at the beginning would not reflect the possible results at the end.
This could also later be used to add more types of 4* troops.
I am not thrilled with this implementation, especially its resource cost and real world time, but no queue for retraining may actually be a feature and not a bug.