I've been on a team with someone who basically does this -- spent half the day not working, usually selected a single low-difficulty task at a time from Jira to drag out over a couple days or a week, and often went home having put 30-45 minutes less time in per day than others on the team.
There was an unspoken but very palpable dissatisfaction from everyone else on the team. No one wanted to be the Bad Guy that calls them out on it, but the manager was unwilling to push them to do more, and a result they were able to get away with slacking off while others were actually putting in effort.
It was tremendously bad for morale. That's the biggest effect it had. All the other team members were bitter about the manager's unwillingness to keep the team on a level playing field. If you let this slide, you're ultimately letting everyone else on the team down.
The point absolutely shouldn't be to police someone's time; it should be to ensure that, if they have time to slack off, they should be pushed to take on more tasks during sprint planning. "That seems like it won't take too long, can you take on these two tasks as well?" is really all it takes.