As noted in PR27341, support for DW_TAG_generic_subrange is missing.
This went in: commit df7a7bdd9766adebc6b117c31bc617d81c1efd43 Author: rupothar <rupesh.potharla@amd.com> Date: Thu Mar 17 18:56:23 2022 +0000 gdb: add support for Fortran's ASSUMED RANK arrays ... and seems to implement this. I'm closing the bug.