"This account is years old and never went into "Uncategorized Transactions" on the home page before."
"Before", when you elected to display All Accounts in the See How Where Your Money Goes widget on the Home tab, no investment accounts were selected. Now when All Accounts are selected in See Where Your Money Goes, all investment accounts are selected.
"What should I do?"
Change the accounts selected in the "Options" dropdown for See Where Your Money Goes: elect "Custom ..." and choose only the accounts you want to see.
[It might be worthwhile to create a new IDEA discussion here to suggest that transactions which can never have categories, should probably not appear in features which are telling users they have uncategorized transactions.]