Time time format is 12h after adding the widget but in the setting the 24h checkbox is checked. You need to uncheck the checkbox, click [apply], check again, [apply] and after time format changes to 24h. Reproducible: Always
Are you sure it is checked? Here it is "intermediate" (ie. just half-filled) which denotes the default of "follow system locale".
https://youtu.be/52qhuC0W58M
Yeah I can now see the checkbox is half-checked and from the perspective of intuition this sucks. :) I cannot normally get the half-checked state and this looks like a bug.
The odd intermediate state UI was changed a few Plasma releases ago (5.18 IIRC?) to be a three-item combobox, so this should no longer be a problem.