[SalesForce] Welcome Email not getting sent to community users on register

This was an issue on a project I worked on as well. This particular feature is not very admin friendly (as you found out the hard way:). We chose to uncheck the "Send welcome email" box under Communities > Administration > Emails. We then created a workflow rule on the User object to send out the welcome email to new community users. The workflow rule fires on record creation only as has a criteria for "Profile EQUALS 'Your Community Profile Name(s)'".

I know this is old and might be pretty basic, but if the above steps did not work, check that the profile the user has is associated with the community.

From Setup | Feature Settings | Communities | All communities.

Select Workspaces next to the Community where you want to add the user as a Community Member

Select Administration | Members | Select the user's Profile | Click on Add. Click Save.

Ref: https://help.salesforce.com/articleView?id=000316118&type=1&mode=1. ==> I know it's a reset password help link, but the idea is the same for receiving all emails, the Profile needs to be available in the community or else no emails will be triggered.