As we pretty much understand each other I'll ignore the minor differences.
However:
What seems to happen with some leaders is that they don't seem very in control of where their own weight is. They may start out with their weight forward over the ball of the foot/instep...but it's like when they start walking their weight shifts back to over their heels. And, yes, just that small shift can make a difference...particularly if you're dancing with a lot of forward intention/shared weight. Then they arrive at their next step, but the follower is left in an extremely awkward in-between place. That's my theory based on what I've felt anyway.
This has triggered some further thought about . . . . . . . shoes!!!
I've written elsewhere about the significance of heel height for forward
projection and stability but not about the dynamic effect of an effective
heel height and the consequences of a low heel.
People seem to dance in all sorts of shoes without much regard or appreciation
of what's needed. And they seem to be sold in much the same way.
If the heel height is too low the leader's weight will be biased back when standing
or moving weight forward makes the heels go light, probably weightless,
and stability in the stance is lost. Walk in such shoes and tipping/moving forward
is possible if a little unstable but when the weight is landed, the heel isn't
where it should be so the leader's weight could indeed sink onto the heel
under the follower's forward intention and destroy the leader's.
It could be the effect Zoopsia is feeling.
In my case a good friend and tango partner used to tell me about a hesitation
she felt but never was able to exactly tell me what it was. Another felt an
improvement when I solved the shoe dilemma and was able to connect
firmly with the floor. Perhaps this has something to do with it?
Your example of the weight shifting back onto their heels at the start of the walk
is a classic example of the heel not being as high as it should be. Care is needed
as my early experiments resulted in heels that were too high and you end up
sort of oddly trying to walk
over your heel. This ain't easy!
It's not surprising that men use all sorts of different shoes, they are never
told and it takes a lot of (over?) analysis to get to this understanding.
Shoes of all sorts of weird shapes and constructions are sold/bought.
I looked at three different designs of Darcos shoes recently, every one
was different, only one possibly suitable in my eyes for tango. They
couldn't all be right as they were all so different. The most weird was an
eye-appealing black and red. The sole arched as normal from the heel
but at the point of contact of the sole the table/ground, it was bent
sharply upwards at about 20/25% as if shooting for the sky.
Not sure it was suitable for a foot never mind Tango!
I know this doesn't help directly but have a look yourself at those
leaders' feet and stance next time you're sat watching.
News of your views would be welcome.
Meanwhile I have been thinking of posting an article to gather up
these shoe posts as there is so little information about.
I may give this some more thought in time.