Sprout Email variables in custom fields aren’t replaced after update
This was a change introduced in v4.2.0 to address a security vulnerability. It was noted in the changelog and upgrade notes.
The Subject and Default Body will still render values dynamically but any personalization needed for custom fields would currently, as you've discovered, need to be handled in the templates.
I do have plans to review this more closely and see if I can find a workaround for custom fields, but for the time being, had to revert to the support above for security considerations.