I’m not sure what’s going on here. Chrome says the certificate for forums.puri.sm (issued by Let’s Encrypt) is valid, yet the cert details show it expired in October.
This looks like a caching issue on your side; the certificate has been updated on 2018-11-14 and is valid until 2019-02-12.
This looks to me like a Microsoft Windows dialog, not a Chrome dialog (though I’m not sure, since I don’t personally use either), so I’d guess that Chrome validates the certificates itself (and correctly), while the dialog shows you outdated information from the Windows certificate cache.
I’d say you’re right about the behavior. Yes, that’s a Windows dialog but that’s what’s referred to when you ask Chrome for more cert details. It was my work PC, so I’m stuck with that combo there for now. EDIT: Tested from Android and I see the correct expiration date there (Feb 11, 2019), so yeah Win/Chrome has some weird caching bug.