That’s not quite what was happening. Any member - suspended or otherwise - can log into their Premium account, and it doesn’t register on the forum “last seen” time; only an actual log-in or attempted log-in at the forums does that.
A suspended account making a log-in attempt at the forums will register an updated “last seen” time and a new “Last IP address” (if different), even though they never reached the forums.
We also established that suspended members were unable to log in, but were not shown any message stating their account was suspended. We have no way of testing whether this is simply a problem with our own set-up, or a wider issue with SSO.