Sometimes, even though a team has a good performance, there are things that are missing from their view. It's possible that Milan is currently ahead of Juventus in the standings, but as you said, they have conceded so many goals and that is a serious problem, especially for a big team. Maybe even though Milan often concede goals, they have a reliable squad so this is not a problem because they can always be productive.
But the most fearful thing is if a player experiences a decline in performance and this is caused by many factors. It seems like Milan doesn't want that to happen and they have to avoid this problem as much as possible. Well, defensive players are one of the important keys so that they don't concede too many goals because the defense also has to be strong even though their attack line is always productive. I think Pioli is also preparing a list of players they are ready to recruit at the end of the season.
Actually AC Milan is just three points away from Juventus which I don't see to really be a big deal in that Juventus can still be able to meet up or probably displace them depending on whatever the case maybe, if Juventus gets to win their next game they will be at equal points with AC Milan except if AC Milan aswell gets to win their own next fixture too but if it happens that they draw or win then Juventus will still be behind them.
Milan will actually try their best to do everything in their power to keep the team at the top because they would want to make sure they are still fit for the Europa because they also have the Europa league game to tackle too after the serie A games and so their form will most definitely not be same as that of Juventus who has got only the serie A to play, it's very possible that Juventus may get ahead of AC Milan subsequently if they improve their performance and as they have got only the serie A to play but then AC Milan still have the Europa to play too.
The fact that AC Milan has conceded too many goals is actually a problem because since they are looking like they are going side by side with Juventus, if they happen to get a tie, they would use the goal difference to position them but then it's going to be in the favour of Juventus in that they will have lesser goals but AC Milan has concede too many goals already which is actually detrimental to their position on the table.