There is definitely some weird stuff going on here, although I'm not sure how much of it (if any) is new or not.
The first thing is that it *mostly* doesn't matter whether you are creating a cut/extrusion for the first time or editing an existing one. What matters is what the feature extent (distance) is set to. If the extent is set to zero, then you can add or remove sketch profiles without using the <ctrl> key. This would generally be the case when you first start a cut/extrusion. But as soon as you set the extent; either a distance, up to, or through all; you can't add or remove sketch profiles unless you hold down the <ctrl> key. This I think is where the confusion starts, because it's not immediately obvious what is actually triggering you to have to use the <ctrl> key. I don't do much modelling in Fusion (mostly just CAM) so I had to mess around a bit to figure this out. Coming from Solidworks (where you can select sketch profiles without ever using the <ctrl> key) this feels a little odd, but now that I understand it it makes more sense. Or at least it would if it was ALWAYS true.
However the exception to the above (and why I said *mostly*) is that when you go to edit an existing extrusion it will let you add one (and one only) additional sketch profile without using the <ctrl> key, even though the extent is set to somethign other than zero. This seriously adds to the confusion and I would guess is probably unintended (and likely a bug).
The next bit of odditiy is that when selecting sketch profiles you cannot select, unselect, and then reselect a given sketch profile without the mouse crossing over a sketch boundary first. This one is is a little hard to describe so I'll show a video. Here I'm not sure if this is intended behavior or not. It's strange but it doesn't seem like something that you would want to do intentionally as a user so... not sure.
http://autode.sk/22siAjr
EDIT: Just saw Phil posted right before I did. I think he got most of what I just said except for the select, unselect, and reselect thing.
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