ArcMap 10.6.1. Opened my normal editing mxd, added a hosted feature layer from my AGOL organization (using admin credentials), start editing on SDE data in the same map......
I see no difference if the hosted feature layer is loaded or not, turned on or not. Doesn't seem to have any impact.
You might try exporting to FGDB, transfer to local machine and do Geometry check(s) on it. Maybe there is something "wrong" with the HFL....
Tested using both standard FC's and also geometric network FC's.
R_