If you select the declaration of local a, its usages get properly highlighted:

But if you highlight the usage of a in a tuple, then its declaration is not properly highlighted:

I think the root problem is that when you highlight a member of a tuple (including one on the left of a deconstruction), there are two views you could take: (1) you're pointing at the expression of that member, or (2) you're pointing at a member of the tuple.

Currently, QuickInfo takes the second view (see screenshots below).

There is a similar problem with anonymous types:

FYI @CyrusNajmabadi

Relates to #20116