Came here to say it is happening by on our end too. Received a lot of expired ssl cert notifications, but cannot reproduce it. Currently trying to understand why this is happening. At first sight seems a glitch.
You are either sending and explicitly trusting a full chain up to AddTrust, or something of the sort. Or you might still have expired AddTrust in your ca-certificates bundle.
I haven't seen this issue reproducible in any modern browser, but it's been annoying with explicitly-defined trust stores in some old apps.
I haven't seen this issue reproducible in any modern browser, but it's been annoying with explicitly-defined trust stores in some old apps.