Webflow's native form handler has some limitations;
Fortunately, there are alternatives which do not have these issues, and which can add a variety of features.
Here's an excellent list of form handlers;
https://jamstacktools.org/browse/all/form
Sygnal's favorite, it is low-cost, and can be used across multiple clients with a single account. It also has excellent anti-SPAM capabilities and file upload support.
Formspark offers the lowest cost option with a free plan and a one-time payment option.
https://documentation.formspark.io/
https://formspark.io/pricing/
Offline?
https://formsubmit.co/
Expensive
https://getform.io/
https://getform.io/pricing