When you Alt-Tab out of the game and then back into it, adaptive refresh rate stops working. Monitor stays at max refresh rate regardless of the framerate. To make it work again you have to restart the game.
Video shows the issue:
(green is framerate, orange is monitor's refresh rate, 144hz monitor)
No other game has this problem for me.
It's a Freesync montior (AOC C24G1) with a Nvidia card (Geforce GTX 1070 - latest drivers) using Nvidia's "G-SYNC Compatible" function. Windows 10
If anyone is able to test if the same thing happens with a G-Sync monitor + Nvidia card or Freesync monitor + AMD card, please do.