You may need to check that you did select button and that you did save your changes.
On your dashboard select online store - themes - customise - product pages - settings -product option form - show quantity picker - Button - Save.
Then you should view your online store by opening a new page to view your store and then select a product page. The +1- should show. If not try refreshing the page, F5.
We are always checking on second large monitor with chrome DevTools many resolutions
and before that cache is always flushed and pages completely renewed.
My partner is a JAMstack developer, he knows what he is doing, meaning:
we are not newbies to the development cycle.
Anyways: to now avail.
All dropdowns instead of buttons with a - minus and a + plus
Thanks for the hint, though.