When Messi drops to cm or cdm to get the ball and leaves his cf position.
People think that he is doing that due to a poor midfield, but imo, that's his current "thing".
He wants to be involved and touch the ball all the time. Create/pass/score, yet without too much off the ball movement and running.
He will drop deep forever.
So, if you play Dembele-Messi-Malcom, there is a high chance of having no one in the box too often due to Messi's current style, with 2 wingers close to a touchline.
And when Messi gets the ball, he will be surrounded by 4 defenders who have no one else to mark.
Which can lead to extreme lethalness.
Messi has to adapt like everyone else. Malcom and Dembele most certainly are not going to be touching any touchline. They are creators/support forwards more than anything.