How many accounts do you have? Are they all with the same provider? Do they all behave in the same way?

I ask because I'd suspect the server you're using is part of this problem. I am not seeing other users complaining about this, and my own test messages using Bcc all work properly (apart from a known problem with "Unknown recipients:" being omitted.)

Some email providers do not support Bcc. It's criminal that they don't actually tell you that something you expected to be private will not be treated with the respect it deserves.

I am not aware of any adjustments in Thunderbird that would disable Bcc. That's why I think your problem is due to an external cause.

Is this a hotmail/live/outlook.com/msn account? There were reports a few weeks ago that BCC was no longer supported if the account was accessed through a 3rd-party program such as TB, but it still worked if the account was accessed through webmail. I believe MS was notified of the issue, but I'm not sure if it has been resolved.

https://support.mozilla.org/en-US/questions/1134989

Sorted thank you. Looks like an update and change of display format caused the confusion for me.

Thanks. Have sorted it now.