we have (4) 6" fans, (1) 10" fan, and (1) 20" box fan
that we run from inverter power, NONE of them squeal, not on the 300w go power psw , or on the 1250w tripp lite power verter msw
the 10" is 7yrs old and ran two summers on a vector 400w msw from pep boys
and did not squeal then either (that i remember)
try a different Fan..
this might be specific to the windings in the fans motors
or specific to this inverter
as far as Anomaly..
there is NO anomaly
most inverters today are built with a floating neutral
aka not bonded..it is normal and it is safe in the RV
i myself do not want a bonded electrical system when dry camping, NOT connected to shore power, i do not desire having the frame or metal skin of my RV, as part of the active hot electrical system, by being tied to the neutral , even more so for a portable generator sitting on the ground, possibly creating a circuit path between damp ground and the rv body
i prefer to keep the real power between neutral and hot, not body frame and hot,, with a floating neutral,, the perceived 60v to grd/frame is induced phantom voltage with NO current potential because it is not electrically connected to the power producing inverter