What may also be transpiring here is loose stuff in the pre filter is seriously restricting the flow as it increases at higher rpms. As flow increases it moves or presses contaminates or junk tighter to the filter medium.

Anyways almost always filters have to be eliminated first in any scenario that seems fuel related. I might also dump the contents of the secondary fuel into a glass container looking for water etc. perhaps. Even though it is also being replaced.

Bulk carriers of fuel do not always properly clean their tanks out before adding diesel fuel. You can never be sure what their last load was. This to me is why with any diesel it is a good ideal to always have a set of spare filters in the trunk and the few tools required to change them.

I had a case years ago that it appeared some bunker c oil was in the customers gas tank. It could have been anything as we did not send a sample out to a lab. The only thing for certain it was the cause of the operational difficulties with the vehicle. I just threw in bunker c as that's what it looked like. Probably not though as it should have dissolved in the gas I suspected if it was. Maybe not.