Transistor Ft as function of Vce?

[Dave Zan]

To build the perfect amplifier that we all want - we need to know how transistor Ft varies with Vce. So we can optimise.

Bob Cordell's book (p22) says Ft droop at lower Vce at hi current, with a "Vce ... as little as 5V ... current ... several amperes" in an example.
The On semiconductor datasheet for the MJL4281A (fig.9) has the reverse, with better Ft at Vce=5 than Vce=10 for the entire current range shown (.1A to 10A)
A net search didn't show much accessible research. I even overworked my brain with the original Gummel_Poon paper in the Bell Systems Journal. I suspect there's several different effects that interact in a complicated way. Any pointers to more information or an explanation?
Or it could just be a mistake in the datasheet.
Which is what it is.