It looks like a change was made to the meta-box plugin a few months ago to force the taxonomy field parameter to be an array. The admin columns plugin, however, expects this parameter to be a single string value when checking for the filterable setting. This is preventing the dropdowns from appearing at the top of the admin grids.
See this git commit for the change: https://github.com/wpmetabox/meta-box/commit/b894acc58d587a8274c7b970e4951cb77d16b19e
Thanks!
