Raglan sleeve, just had a discussion about the pulling, gathering and wrinkles that are common with raglan sleeves, in the armpit area, both in the bodice and sleeves.
Current raglan is made for better arm movement, for sports and activities, not just style.
A few ways to minimize them, is to use better fabric, with better stretch and rebound, most people use a jersey knit to make them, sometimes jersey does have of ok to good stretch, but a little lacking in rebound, and usually hangs off the body better in the lengthwise up and down, vs crossbody.
The easy fix is better fabric.
With a traditional cut of the pattern for a raglan shirt, you are asking a lot of the fabric, the bodice is one piece and sleeve is one piece.
For the bodice that is one seam going from neckline to middle side seam, on a curved body with a straight long seam.
A couple of fixes, first modify the armhole, do a bit of measuring and usually make the armhole a bit smaller. I know most do not want to redraft an armhole.
Best thing to do is to go from a one-piece front and back bodice to add a side panel instead of the one side seam. This relieves some of the curving stress on the bodice pieces.
Second is the fit the arm sleeve a bit tighter to the body.
Below image shows a couple of examples of adding in a side panel.