Mailers detached from Google option

We complete this client’s site a year ago, set up with WP Mail SMTP. No problems.
Recently client got the error “cookie nonce is invalid” plus no email delivery (while on a phone).
When I looked inside WP admin, the Mailers section had defaulted back to PHP, which we never use. We use your plugin to validate through dedicated GMail account.

Is there some way of diagnosing this? I have never received an email error message from the site. The plugin self-test works fine.

I told my manager that the recent 5.5.2/5.5.3 might have played a part, but I don’t have evidence for that.

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