Same, Im getting the issue, it's consistently booting me back to the Main Menu after a lengthy loading screen, this specifically started happening to me after the August 1st update, Sandbox and Spectating take a while load but they dont boot me back to the menu, I would also like to add that before August 1st these loading times werent as lengthy, I suspect the game kicks me back to the menu in either occasion due to the Timeout settings, similarly to TF2 and CS2, if the game server doesnt get connection messages back from you in a while, it simply kicks you due to timeout, it seems the Loading screens are too lengthy and therefore the game being in that frozen state for too long gets you kicked from matches due to timeout