There are two different history stacks at play here. Undo/redo works inside of text fields (such as the note pad), but it also works on bigger action in Tropy (such as creating, deleting, merging items, and so on).
For example, let's say you create a note, then work inside the note for 25 minutes. The note will be saved continuously while you're working on it; if you don't do anything else in Tropy in that time (e.g. create or edit other objects) all these saves will be combined together in Tropy's undo history. Therefore, if you leave the note editor after 25 minutes, then undo, it will undo all the recent changes (redo will bring them back). Granular undo/redo inside the note, only works while you're actually editing it.