The FacetGroupComponent view component has a hard-coded value for the classes:
class: 'facets sidenav facets-toggleable-md'

This makes the facets collapse under a hamburger menu toggle below the large breakpoint (< 992px by default).

This works OK if you use the default col-lg-3 sidebar + col-lg-9 main content layout. But if you are overriding Blacklight's (useful) layout helpers to specify different classes (e.g., to keep the facets sidebar visible at the md or sm breakpoints), you'd also need to change facets-toggleable-md to be -sm or -xs.