Looking into this, there is a few ways we could go.

• While this fixes the common case, it allows multiple geometry types to be enabled at once (holding shift), as it happens this is supported internally.
  
  The only issue with this is it allows none of the buttons to be pressed too (that could be fixed using a set callback to ensure the value can't have all the geometry bits cleared).

• If we just want to keep behavior as-is (while fixing the bug).
  
  We could have get/set functions for this enum which mask out the bits we don't want to change.
  
  This would be an acceptable fix.

• We could support limiting the bits with RNA enums more generally, by adding (e.g. RNA_def_property_enum_bitmask).
  
  This way the fix would be a one liner:
  
  RNA_def_property_enum_bitmask(prop, MOD_WVG_PROXIMITY_GEOM_VERTS | MOD_WVG_PROXIMITY_GEOM_EDGES | MOD_WVG_PROXIMITY_GEOM_FACES);
  
  This removes the need for rna_UserDef_studiolight_type_get, rna_ShrinkwrapModifier_face_cull_get, rna_SoftBodySettings_new_aero_get... (and their set counter parts).

Note, enumbitflags comment is currently misleading and needs to be updated.