Ahh, yes... toggling the checkbox will only remove UNSTUDIED lessons from your account. So if a lesson is in your studied list we won't purge it automatically just because you advance to a higher level.
The only thing I can think of is that perhaps you are behind an ISP that is caching web data? If this is the case the second or third time you request the same URL you will get stale data from your ISP, because the request is never routed to us. If this is the case, one way to debug it is to change the URL of the page the second or third time you load it to include an arbitrary subdomain (i.e. "sjgks.popupchinese.com") and then manually refresh the page. Our server redirects meaningless subdomains to our main site, but the different URL should force your ISP to download the page afresh. If you can see the change in lesson status on the newly downloaded page then it can be confirmed to be a cache issue.