Thanks for reporting this important issue.OK. This is a compatibility issue.WordPress applies a default
tabindex of 100 to the submission button on the default Registration Form; and starts with a
tabindex of 10/20 for the Username/Password that it builds in by default.
So in cases like yours, where there are MANY Custom Registration Fields, s2Member can potentially go beyond the bottom-most
tabindex of 100. This happens because s2Member increments its Custom Registration
tab indexes by 10; leaving room between each Field for custom Hooks/Filters.
So the solution we're going to implement in the next release of s2Member, is to modify the form submission button to a tabindex of 1000, making additional room for site owners with many Custom Registration Fields. This can be handled dynamically via jQuery.
I'm attaching a JavaScript file for you with these changes, so you can go ahead and correct this issue on your installation while you're waiting for the next release of s2Member
( v3.5.8+ ). Incidentally, I'm also including a bug fix in this JavaScript file that corrects the bug you reported
here.
Please upload this file and allow it to override your existing copy./wp-content/plugins/s2member/includes/s2member-min.js( then, clear your browser cache; and/or refresh the page )