Yes, @HughesTooling has got it right. This is an interplay between Fusion's feature/selection and the "remember last value" which was added this summer some time, I think.
Here's the background: Fusion previews the "real" operation on the model. That is, instead of producing some kind of temporary preview, and leaving the existing model in its original state until you hit OK, it actually temporarily executes the operation on the model. But, this creates a problem for us software developers. In some commands, the preview generates new model entities that are really temporary. Or, to say it more accurately, these edges cannot be selected in the same operation as the one that created them. I always use Fillet as an example in explaining this:

Fusion cannot create a fillet that fillets the original edge plus the introduced edge in the same operation.
So, we had to block selection of those "new" edges during the command that creates them. You could argue that "that's just a matter of writing code to identify those new edges and block them". Maybe, maybe not. Let's just say it turned out to be harder than it might seem.
So, we added this notion of "rollback for selection". Where if you hold down CTRL (CMD for those Mac-inclined), we suppress the preview, and let you select more edges. Why CTRL? Well, the justification at the time was "most O/S's have this paradigm where CTRL select adds to the selection set, so if we are going to add, it seems like CTRL is a natural place to put this selection rollback". Let's just say that this has been controversial, even within the Fusion team. This has been an ongoing debate for 4 years or more, with passions nearly that of the "gun rights vs. gun control" debate in the US :). There have been lots of other options proposed, but nothing has ever jumped out at us as "this is the answer!".
Anyway, I digress... How it works today: Any time there is a valid preview in a command, the only way to select more things is by holding down CTRL/CMD. So, the first time you create a Chamfer, the default value is 0. You can select all the edges you want without CTRL. But, as soon as you drag or type a value, then you in that preview state and have to use CTRL to select more. So, say you select 5 edges, and use a value of 5mm, click OK. After the enhancement this summer, that 5mm will now be the new default value. So, if you invoke Chamfer again, as soon as you select the first edge, the command will apply that 5mm value, and then you are in the preview state.
Well, that was a long-winded explanation of a question that was already answered!
Jeff Strater (Fusion development)
Jeff Strater
Engineering Director