If anyone has any doubts about how messed up Marriott's website is, out of curiosity I followed the instructions, except that after logging in, and going to Marriott Rewards Overview as illustrated above, it threw me into a generic page and made me sign in again, literally less than a minute later.

Then I could click on Nights, and get the pop up. The thing is, there was a period of time recently where this function was not working, and as described by the OP, the LT points had gone to zero. Good to see that it's working again, but if LT experience with Marriott teaches you nothing else, it's to try again tomorrow.