Had the same issue with my Mavic Air after a few weeks of light use, I was annoyed enough to call DJI and set up a repair. I sent it and and can you believe they sent me back a brand new remote, obviously un-inspected, as the "new" remote they sent me had the same exact issue, flopping right antenna! I was pissed, called them and wrote them, sent back the "new" replacement for a third replacement and finally got a good one with nice firm stiff antennas that stay in place! Keep in mind if you do decide to send your remote back to DJI it's a slow process! Pick a rainy week! The three back and fourth send/receive I had to deal with took close to a month. Overall I'm happy I did it though, in the end it work out!
In between all the back and fourth with DJI I found these antenna braces and bought a set as a back up, they work great if the problem ever arrises again, it's the perfect fix if you don't want to bother sending your remote back or if you're out of warranty. The link is below,
Stick a DJI Phantom, Mavic, Spark, or Inspire decal on drone case or car window
www.phantomhelp.com