Hi Rob,
For some reason, I believe tabindex is not the preferred method. I solved this issue by changing the way the navigation bar is built. I think I changed the order of elements in the template and forced some element to wait for the others before being added to the DOM by the Javascript. I don't remember the details; I would have to look up the project to find them.
In any case, stay on the lookout for unintended consequences of altering the tabindex.
Note the warnings in MDN:
https://developer.mozilla.org/en-US/docs/Web/HTML/Global_attributes/tabindex
Here is a more detailed article, from several years ago, that discusses this issue:
http://adrianroselli.com/2014/11/dont-use-tabindex-greater-than-0.html