It don't understand "now every transaction that has Split categories goes into that Classification.'
In the account register, you might see if you like the display better after you press Ctrl+T. It is a toggle.
If you are having trouble with reports working the way you want with a Classification, here is a note I made a while back:
If all or none of the particular boxes are selected for the
classification in a report, then transactions without that
classification are displayed. Select a subset, and other
transactions are omitted. So for a given classification, you could
define an explicit member called "null" or whatever. Select all but
that one in a report customization, and the report will only show
transactions that have a value for the classification.
Alternative: to avoid need for bogus/null class, in Customize
SelectAll, and uncheck one classification, and reselect it. Then
those without the classification filled in will not show in the
report.