Right on target, and great reminder about report modifications affecting upgrades.
Assuming this is a from-scratch report, I took your advice and looked at the HTML Configuration dialog to the right of my report table.
Its top-of-dialog context says Applications, but no fields or metrics have been dragged down to the bottom four areas (Filters, Columns, Rows, Values) - so this HTML component doesn't know which object context (if any) to use.
My formula uses Cost and Budget, both of which are metrics.
To set the context for this report component, I found Cost in the Calculations perspective and dragged it down to the Values area:

...and then my formula worked correctly, showing the warning message.
Even better: drag a locked field down to the Values area. This will gray out the object context area (top of configuration dialog) to denote that the HTML component is now locked to the Applications object:

...in the example above, I dragged the Application Name field, which was published in the Service Costing/Applications perspective and locked to the Applications object.
My formula still works correctly, but now I have reduced the chance of inadvertently changing the context next week when I work on this report some more.