This, in turn, leads to total study time (under Stats) reporting an incorrect (too small) time.
That the review has a duration of 0 has also been reported here : “set due date has a weird effect on cards table”, from April 2021, though the main emphasis there was different (I cannot include a clickable link in this post; here is the non-clickable URL: https://forums.ankiweb.net/t/set-due-date-has-a-weird-effect-on-cards-table/9289).
I have just verified this with Anki 2.1.55 Qt 6 under Linux, but I think this has happened for some time and certainly with Anki 2.1.54. I think it is not related to add-ons, since it happens when starting Anki without add-ons (keeping shift pressed).
Answer time is not recorded when manually scheduling cards, since the action can be performed even outside of review, and it isn’t aware of which card may or may not be shown at the time. Ideally we might record the time if the card happens to be shown, but I’m afraid the complexity it would add is hard to justify.
Thank you very much for the detailed explanation and apologies for the noise then. I wasn’t aware that the action could be performed outside review nor that Anki wasn’t aware of which cards were shown at the time. I see it would not be worth dealing with this.
I wonder if it might make sense to point this out in the manual, for example here https://docs.ankiweb.net/browsing.html#cards , with literally the answer you gave me: “Answer time is not recorded when manually scheduling cards, since the action can be performed even outside of review, and it isn’t aware of which card may or may not be shown at the time.”
I just created a pull request with this change (https://github.com/ankitects/anki-manual/pull/126).