I would report this directly to Wix as a bug and include:
- A working older dynamic page where the same setup works.
- A newly created dynamic page where it fails.
- The CMS field key used for the alt text.
- A live page URL.
- A screenshot showing the image source connected to the image field and the Alt text connected to the text field.
- Confirmation that the live page outputs the file name instead of the CMS alt text.