Hi Kari
ATUM doesn't change these stock status labels in the WC product page.
It only adds the list of variation stocks if you configure it that way in ATUM Settings >> General >> "Display stock info for variations". And that should not affect the product stock status shown there.
Are you sure this is being caused by ATUM?
If you disable the "Display stock info for variations" option, works well?
And, if you disable the ATUM plugin works well?
Best Regards.
Salva.