Hi Chiara,
This issue usually occurs when the using an older Connect version to publish the survey and use new version of web app(survey123.arcgis.com>Settings>Versions) to render the survey form.
The problem would solve if you use Connect 3.12 version to republish and use 3.12 version of web app to render the survey.
Could you please try to sync the version of Connect(3.12) to republish the survey and make sure the Settings is also set to the same version (3.12)?