It's not all journals, just some. The two personal journals it happened on are both plus accounts, and the community is a basic. One personal journal it didn't happen on is paid. And when I visit the three journals my basic account got trapped on using this account (which is permanent) there was no trap. I'm tentatively concluding that if you have a paid or permanent account, you never hit the trap. If you have a basic or plus account, and you visit another basic or plus account, you do get trapped.
Is it odd of me to be wondering what the heck they did this for? I thought everyone knew that back-button traps were counter-productive, that the only reason to do it is because you want to trap people on your web page so they'll buy your stuff or click on your ads or whatever, but everyone also knows that people hate these things and pissed off people neither spend money nor click ads. So WTF?
And of course they sprang it on us with no prior notice. That's just SOP for LJ these days, isn't it? [sigh]
Stop digging, guys. Really. Just stop.
[ETA:] I did some more experimenting and found that if you're not logged into LJ at all, you don't hit the trap; you have to be logged in with a Basic or Plus account. Note that I'm assuming the Plus account; I don't have one to experiment with but my Basic account got trapped on two Plus accounts, so I'm assuming the Plus accounts would also get trapped. That might not be the case, though.
I'm on Firefox right now, but since I started up my pro-writing journal, with its own Gmail address, I habitually have both Firefox and Netscape running, with Firefox logged in to AngiePen's LJ and Gmail and Netscape logged in to AngelaBenedetti's LJ and Gmail. I first ran into the trap on Netscape. I just experimented some more and if I log into the AB account with Firefox, I don't get trapped. So what browser you're on is a component. I don't have Internet Explorer anymore or I'd try with that. :/ If someone who does have IE would poke around and report results, I'd be grateful.
If this is just a glitch it's a pretty darned weird one. I'm still suspicious that it only hits the non-paying accounts. [/ETA]