2 finger SDL_MULTIGESTURE events with only 1 finger down

My app is receiving occasional SDL_MULTIGESTURE events with numFingers == 2 even though I have only 1 finger on the trackpad. I am moving this finger around to drive a virtual trackball rotating a 3D scene. As far as I am aware I haven’t done anything except move the 1 finger that is already down when one of these events unexpectedly arrives.

This is on macOS Catalina (10.15.6) using a Magic Trackpad 2.

Is there any way to prevent this? I can’t turn off multi-gestures as I am using them for other actions.