There's a note at the bottom of this help document that clarifies there was a recent change (Summer '19) that is causing your issue
Please note as of Summer Release 19', Users must have a verified email address and .invalid removed from the email address in order to send email alerts via Workflow, Process, Flows or Triggers.
I didn't find any note about it in Summer '19, but did see that Spring '19 introduced the change of format to appending .invalid to user email addresses in sandboxes.
Either way, if you change the user you're using/testing with to have a valid email address then the emails should send out.
Just remember the user has to verify the email change through an email message they receive from Salesforce before the email actually changes