Has your watch just started doing this or has it never hacked (the stopping of the second hand)?
If it's never worked, then it's likely missing the stop lever, it's item 1 below. When you pull the crown out, the short leg is moved by that action and the lever swings about its pivot (the small hole in the junction of the L shape). The long leg then touches the balance and stops the movement. It's that simple and the lever does nothing else. If this lever is missing, it won't hack.
The lever is held down by the auto bridge (the big piece at the top) so it's unlikely to have come free and be floating around in the movement. Doesn't seem likely that it's broken either, unless Al sees this problem regularly, but perhaps it was not refitted at the end of a service? Regardless, in case any of it is loose in there, take it to a Watchmaker who can remove the 3 bridge screws in five minutes and confirm what's wrong.
View attachment 185172
Regards, Chris
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