We're aware of the issue and are currently gathering more information regarding this matter.
Meanwhile, you can use the following workaround:
1. Open the form settings.
2. Click Conditions.
3. Edit the relevant conditions that involve the signature field and uncheck the signature field.
4. Publish your site to see changes live.
This article is credited to the Wix team - https://support.wix.com/ and we thank them for that!