Stroke weight defaults to 1pt when a new type of brush is applied.
To replicate the issue:
Assign a 2pt stroke to an object, then apply any brush to it — the stroke weight becomes 1pt.
Switching between brushes of the same type (Calligraphic, Scatter, Art…) seems to keep the weight intact, but switching the type of brush triggers the stroke weight to default back to 1pt.
Expected result: the assigned weight stays the same regardless of the brushes applied.