I'm listed as the technical contact and have been for 5+ years and also get the regular mails to 'verify contact details'. I did not get the Aug 25 email. I did get the Sep 4 mail from that list ID.

Once I noticed I did not get an email, on Sep 3, I checked some checkboxes at https://my.atlassian.com/email But that page also says tech contacts should always receive an email regardless of settings. I have received other security announcements in the past.

Office 365 can't find any emails from Atlassian on Aug 25 when searching using the Message trace tool (which also includes any spam mails, deleted mails, et cetera), so I would suggest Atlassian fix their mailing list.