Edwin Johnson wrote:

...
Placing the balun (4:1 or whatever) under the twin lead would really do
strange things to the impedance and probably cause the problems you mentioned.

73 ...Edwin, KD5ZLB

Actually, I would expect placing a 4:1 balun under the 300 ohm balanced
line and before the 50 ohm unbalanced to provide a step-up(or step-down,
depending on the "direction" you view it from) of 50:200 or 4:1, as it
properly should. However, as someone presented in a paper a little
while back, some antennas "filled with errors" are able to function in
some manner and end up gaining their supporters ...

If you were to place a 1:1 balun at this same point, I would expect
little difference, but a difference (and, since you are mismatched at
this point, simply maintaining this mismatch with a component
introducing some loss and "redirecting" CM currents, not a good difference!)

It seems the "misunderstood/mystical/magical balun" lives on ...

Regards,
JS