Just off the top of my head...

Where any of these problem formulas copy/pasted into the new workbook
from the test workbook? This would establish a link, I would thing,
causing the new workbook to call the UDF in the test workbook.

Were any of the sheets containing problem formulas copied from the test
workbook into the new workbook?

Is the new workbook being created from a template, OR is it the old
workbook "SavedAs"?

If anything that references a range object in the test workbook is
copied into another workbook, the definition copies as well. This
suggests to me the range objects in the test workbook are globally
defined. If, as you state, the UDFs are in the code module of each
sheet then any range references should be defined as local to that
sheet.

Global range defs travel with copy/paste,
replacing any global defs with the same name.
(Dependant on how the Q asked during paste was answered)

Local range defs are unique to their respective sheet,
and travel with it wherever it goes.

--
Garry

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