Same problem. For me the issue only shows up in Firefox, the Chrome extension works as expected. I tried all recommended solutions with the latest version as of today (93.0 64-bit on MacOS 11.5.2), without success. I had this problem for several months, and finally decided to report after my vault accidentally showed up during a video conferencing session.
Side note: I tried first to contact support via email since I have a personal Premium membership and I'm entitled to that. Fun fact, after I log in the contact form only shows the "Call me" option. I will report in a separate ticket, but I'm thinking that perhaps it's time to abandon LastPass for good and switch to a different company.
My wife and I switched to bitwarden months ago. It has been so completely problem and bug free since that I haven't even thought about it, which is exactly how it should be. It's a real shame because lastpass used to be SO GOOD.
Yup, I have the same problem on Chrome. The "Open vault after login" option seems to have no difference whether or not its checked. When I'm not logged in and click on a password form, it opens a new tab and has me log in, then brings me to my vault (like everyone else, this is just wasting my time 90% of the time). I have also switched the option to open up pages in the current tab in case that helped (it didn't). What it should do is have a login view come up via the extension rather than opening up a new tab. But it sounds like Lastpass is on a downward trend? That's unfortunate to hear.
TBH I don't know why our company is using Lastpass when there are open source alternatives like Keepass. Closed source isn't ideal for security purposes.