Inaccurate Current Usage for Matrix > Asset fields and Matrix > Supertable > Asset fields

[howells]

I might be missing something here but the "Current Usage" for practically all assets on a site that uses matrix fields extensively shows "Unused". The site is a multi-site set up but whether it's the default site, or another, doesn't seem to have an affect (I'm using 2.3.1).

Two examples here but this applies throughout -

Via an asset field within Supertable, within a Matrix:

Via an asset field, within a Matrix: