When I insert a Number annotation and undo it, the “Number Seed” does not decrease. For example, if I insert a Number annotation 1, then undo it, then click to insert a new Number annotation, it will contain the number 2.
Having it insert a Number with the same value as the Number annotation that was undone would be useful to re-add the Number somewhere else.
The workaround for now is to decrease the Number Seed in the toolbar after undoing a Number annotation.

(This bug is from the downstream project Spectacle, the KDE screenshot tool. https://bugs.kde.org/show_bug.cgi?id=440395)