Adding custom ui.undo on panel.character or on panel.fieldediting or on op(‘field/text’).curText changes cause issues with the undo stack. Long story short but ui.undo callbacks do not function as expected when implemented while field.panel.focus == 1.
It appears that fields have their own undo/redo functionality while focused, separate from the main undo stack. While focused undo acts like redo after one undo is executed and then toggles back and forth between undo and redo. ie. ctrl-z toggles back and forth between two states while focused