Hello Lody,
Welcome to the Prismic community and thank you for reaching out to us.
This is a known issue. Our product team will work and this will be fixed in future versions.
Workaround: The workaround now is to add the alt tags in the media library before inserting them in the rich text field or you can type the alt text directly in the field.
Sorry for the inconvenience.
Thanks,
Priyanka