Caused by {rB841df831e89d} and {rB3558bb8eae75}.
These commits moved flags from `eLineArtGPencilModifierFlags` to
`eLineartMainFlags`, but later on in code, these were still evaluated
from the modifiers `flags` (instead of `calculation_flags`).
This resulted in a false condition (`match_output` was assumed true but
it wasnt), leading to a wrong codepath taken.
This is now corrected (`calculation_flags` need to be passed around for
this as well).
Maniphest Tasks: T103887
Differential Revision: https://developer.blender.org/D17062
6aa29549e8