An email is being sent (which is a different issue), but not the one welcoming new members, as programmed into the paid plan.

Which CBSubs email exactly is not sending? Are the normal registration emails working?

I also changed mail provider from Amazon SES to Elastic Email, but this isn't the issue.

How do you know this isn't the issue? You've completely changed your mail server and emails aren't working now. Are your mailer settings 100% correct within Joomla global configuration? Have you tested using the Mass Mailer within CB > User Management to ensure outbound emails are working at all? You should test one to your self and a test account with an external domain (e.g. gmail, yahoo, etc..).

It is a plan Email workflow texts email.

Please be more specific there are 7 different emails CBSubs can send under the Presentation tab of your plans. If using CBSubs Emails integration please also be sure to provide which email within it you are using.

Retested the "Email on first activation" email and appears working fine. Are you sure your install is completely up to date? Have you tested this email with a test user or are going entirely on the reports of users? If only going off the reports of users it's possible it's just being sent to their spam box or your mail server maybe blocking it in which case I recommend adjusting the contents of the email.