Embedding channel iframe results in improper CSP

I am attempting to embed a channel in an iframe (I understand the risks) but am constantly met with this error in my browser’s console: Refused to frame ‘https://mattermost.lavendermedia.club/’ because an ancestor violates the following Content Security Policy directive: “frame-ancestors ‘self’ https://mattermost.lavendermedia.club https://teams.microsoft.com”.

I have tried: removing all plugins and the Teams plugin (both in the GUI and manually in a Linux bash shell) and setting an explicit CSP statement in my Apache virtual host file: Header always set Content-Security-Policy “frame-ancestors ‘self’ https://mattermost.lavendermedia.club https://teams.microsoft.com

Despite my efforts, the error persists which is preventing the iframe from loading with an error of “refused to connect”.

Your help is appreciated in advance,

Ian