This is a SendGrid issue. SendGrid is the default email provider that sends all Bubble-based emails. I HIGHLY recommend switching to Postmark, or something similar. Here is a good forum post about this topic:
You can send password resets and default bubble emails, as shown here:
I have figured this out — I just once deleted the Subject input then re-input the exact same thing (can be anything different), then it worked for some unknown reason.
Tried this because, checking with Debugger’s Step by step feature, the Subject input was shown as ‘empty’.