Curious if this is a normal thing,
I experienced an odd issue with Acorn during development of some blocks. We had a typo in the ACF composer block class, but instead of showing the expected syntax error on the site, it instead came up with the WP error page (the blank page with white box) stating that Acorn must be installed to use this theme. Maybe it is a Sage thing, maybe it is an ACF Composer thing, but since it mentioned Acorn and it was installed I thought I’d start here.
My gut makes me think that something using Acorn like ACF composer failed early but caught the error, preventing Acorn from initializing, and then triggering the Sage error about not having Acorn installed.