We recently upgraded from Civi 4.6.23 to 5.3.2. The upgrade has gone fairly smoothly, but today we discovered a problem where the Member Since and Start Dates are not being set for any new memberships started since the day we upgraded. Though these members are being giving the status of "Current", they are not being given correct access to our site.
I've checked our error logs, and there are no errors. I also checked the database, and the database does not have a Member Since or Start Date, and has no record of a membership in the member_log table.