As of now, Next-Gen handlers may create requests in service desk, if only summary and description are mandatory on the portal. (like JSD itself).
the point is to use Set Field Actions to prefill the mandatory and optional fields with default values when SD requests are created.
This will make sure that all required fields are set and automations may be executed correctly.
Create a customer portal with mandatory fields of this type:
Text custom field
Create a mail handler that sets these fields.
Send emails and check if the fields are correctly set.
Add another mandatory field but do not add it to the mail handler.
Send email, and check if the Incoming Mail Log shows the name of the missing field in the Failed item.
Check the functionality in Jira 7.5.0 and SD 3.5.0 and in 8.8.0 with JSD 4.8.0