A subsection of Second-generation engine in Wikipedia's article on RocketMotorTwo says:
New fuel formulation
Rather than use rubber-based HTPB in the solid portion of the hybrid rocket motor—which had experienced serious engine stability issues on firings longer than approximately 20 seconds with the first-generation engine—the Virgin Galactic-developed SS2 hybrid rocket engine would now use thermoplastic polyamide (i.e., nylon) as the solid fuel component of the propellant. The plastic fuel was projected to have better performance (by several unspecified measures) and was expected to allow SpaceShipTwo to make flights to a higher altitude.
The first test flight with the polyamide formulation ended with an anomaly that was attributed to factors unrelated to this change of propellant, but now Virgin Galactic has switched back to HTPB for the most recent flight per this answer and sources within.
According to the Spaceflight Now article Virgin Galactic completes first rocket-powered test flight since 2014:
Engineers also changed the SpaceShipTwo rocket motor design from one that used plastic-based fuel to one that burns rubber-based fuel. Virgin Galactic previously used rubber fuel in the SpaceShipTwo motor, but switched to the plastic-based compound before the 2014 crash. The propulsion system was not a factor in the accident.
Question: Why did Virgin Galactic switch back to HTPB after one launch using thermoplastic polyamide (i.e. nylon)?