found the difference: caused by flats
in ders_simp, the alts is at top most level , so no fuse and bits stay at the alts level
whereas in ders + singele simp, the alts that should be the final top-level alts is not at the topmost level initially before simplification
so it is opened up and bits fused. later it finds out itself the top level only aalts remaining, but the fuse is not reversible
we do not know what happened either.