I have heard/seen people talk about the load becoming soooo inductive that the lag angle becomes >90 degrees
Obviously that is absolute rubbish!

But we do talk about reverse power flow.
Some would then say that power flow is towards the source or the source has negative resistance
Again, absolute rubbish!

There is a fundamental principle that energy can ONLY flow from the source to the consumer
I can accept that a load like a motor may suddenly be driven by the inertia of the mechanical load to become a generator back to other loads on the grid … but it is no longer the load end, it is the source end, so the overall flow of power has reversed in that instance.
Equally in a multi-source, multi-load grid, any particular line can experience energy transfer in either direction.

So what IS reverse power flow?????????????

Well, this spreadsheet explains it as simply related to the polarity of the CTs at the two ends of a line.

Yes, … it depends on what you have deemed as "forward" power flow versus "reverse" power flow direction!!!

You can select resistive, inductive or capacitive load in the yellow cell.
We can see that the operating P:Q position is in the opposite quadrant at each end of the line! i.e. although the net transfer of energy is in one direction, our point of view at each end would say one direction is reversed compared to the other.
… and again as we would expect, the mathematics prove that PF is positive at both ends of the line despite a negative P at one end!
We can see that full 360^o cos(φ) meters need to have a range from -1 to 0 to +1
However Power Factor Meters will only read in the range 0 to +1, despite operating in the four quadrants as lag or lead.

So, again as we would expect, this proves that statements such as “negative PF only means reverse power flow” is fundamentally wrong, simply because Negative PF can’t exist.

However, negative P or a negative cosPhi (with Phi being angle of I with respect to V) - which is not the same as PF in all circumstances - means that the Source is in front of the arbitrary forward power direction determined by the CT polarity.

THEN, that deals with the conundrum of the idea that reverse power flow means negative resistance … that is just a mathematical "equivalent" based on "reverse engineering" R = P/I² … negative resistance don’t exist in real life either, but it can mathematically!