BCC not sent

I wonder if anyone else has experienced this:

I have all of my identities within TB configured to place a copy of sent emails in the "Sent" folder and also to BCC the sender email address. This has been the case for literally years. I've made no configuration changes at all for a year or more.

Recently I've noticed that some sent emails are not BCCd back to me. If I look at copies in the "Sent" folder, they normally show that the email has been BCCd back to me but in these odd cases there is no BCC line (unless I have explicitly BCCd the email to any other address).

I haven't been able to establish any pattern. An email will fail to BCC then the next one sent using the same identity will BCC just fine. It doesn't seem to be specific to any particular SMTP server (I use two different ones routinely) or ID.