Newly added and OTP-verified email recipient in an action group only gets test notifications, not real alerts

I have a log search alert rule on Application Insights. It sends to an action group with two email recipients:

• Recipient A: in the action group for several weeks. It receives alert notifications normally.
• Recipient B: added recently through Terraform (azurerm provider). I completed the OTP email verification, and the portal shows it as Verified.

What happens:

• The alert fires. The alert History tab shows the action group was triggered, and Suppression status is None.
• Recipient A gets the email, but recipient B doesn't.
• "Test action group" sends test emails to recipient B, and they arrive every time.

Already checked:

• no alert processing rules,
• the correct action group is attached to the rule,
• both recipients are Verified,
• no rate limiting (very low alert volume),
• nothing in spam or quarantine,
• no unsubscribe.

We saw something similar in a second environment: a verified recipient (a Slack channel email address) received test notifications but missed two real alert notifications (fired and resolved). The same address received real alerts from another action group that day.

Questions:

1. After OTP verification, is there a delay or an extra step before a new recipient starts receiving real alert notifications, as opposed to test notifications?
2. Does adding a recipient through the API or Terraform, instead of the portal, affect OTP enforcement or delivery?
3. Is there a known issue with alert email delivery since the OTP verification rollout?

I'm on a Basic support plan, so I can't open a technical support request.