To note is that this was at the end of the craft, when max quality with Byregots was already guaranteed.
In this case, this is the expected behavior.
When Crafting Solver detects a 100% finisher, it removes Observe from the solution map to reduce the number of steps. However, the general algorithm still tries to utilize the leftover CP. This is a minor issue that does not have an easy fix, therefore it will remain this way.
We apologize for inconvenience.
If you wish to avoid this issue, you can disable 100% routes in Advanced Settings.