Bump, I contacted support having the same exact problem as OP mentioned, and this is what they ultimately deduced based on what I was experiencing:
Judging by the information on your account the issue appears to be related to your graphics card (and possibly your CPU). When you are playing with other Tenno, especially when you are hosted by them, your system loading times are compared to the other players with you on a regular basis. If the two systems fall too far out of synchronization then you will automatically be dropped from the game or loading screen. This can also occur because of temperatures rising too high in your system, as the two often go hand in hand. Unfortunately there are not many options for us to provide you with, other than turning down graphical options, cleaning your computer, shutting down background processes that are not needed and changing your settings to laptop mode (you do not need a laptop to do this.)"
Likewise as you, I'm using a moderately potato PC, but I've never had an issue with connecting to previous squads before Buried Debts. Now 90% of the time I'll get disconnected when trying to join a squad. The odd thing is that when others connect to me, they have no issue staying connected and most of them report zero latency, which leads me to believe there's something broken with warframe's matchmaking server.