With 5.5.x they amended the metabox toggle bars used throughout the admin dashboard and changed around the html structure, we will need to try matching.

See example commits at WebDevStudios/custom-post-type-ui@a138839 for changes made by me with CPTUI.

Since I doubt we want to abandon pre 5.5.x users just yet, we'll need to do a version check and probably duplicate CMB2_Hookup->context_box_title_markup_open().

Given how many others have embedded and use our library, I am wondering if we're going to get increased support around the issue from anyone still using legacy bundled copies.