Rounding glitch after 999 when using API to add template rows (when using group separator)

I have a button linked to an API to add a collection of rows. Some data is added to columns automatically (the user makes number entry, it sits in their user row, then gets pulled by the API into the destination sheet). Weirdest thing: up to 999 I get 999, over 999 I get it rounded to nearest 10. I checked the columns, and if either or both are set to use a group separator then the bug persists; if I remove group separator from both then the bug disappears.

This thread has been escalated. Support will follow up via the Messenger in the Builder or through email.