Happen with CC a lot too. It's not that big an issue though.
Global chat and the General Chat channel are different things. You should stay in Gen chat when you return. Global chat turns us off at random. It was useful until you had to turn it back on.
If you run more than 1 EC instance of UO, and both accounts have saving journal active, you will frequently find one account being kicked out of General Chat when you log out (and hence need to rejoin when you log in). I turned my alt account's journal off, since she is rarely online when my main isn't, anyway.
If you visit the online store in EC, it often does weird things to both General Chat and Global Chat when you exit the store. You can't speak when visiting the store, so maybe there is a bug related to that. For example, after exiting the store, I can look at my channels and it says that I am in General Chat. If I click to join it, it says I already am. However, sometimes I am not - and if I relog, it will show None when I look. I don't visit the online store often, but I usually do a quick relog afterwards.
Incognito spell and the Disguise Kit also kicks you out of General Chat, but I think that is both clients and is working as intended.
I have come to terms with these bugs, however. Both clients have their issues, and these are minor enough that I can work around them and benefit from what EC has to offer. When I log in, I take a couple seconds to check my chat channel and my global list, and make sure both are on. Most will stay on for the duration after that, altho global likes to shut down randomly for everyone at times. It would be nice if they got fixed... but there are so many larger, more game-effecting bugs that I would rather they focus on.