Autothrottle Failed, Or Maybe Not

While cruising during a recent flight, I decided to pass the time by looking at the failures menu in the FMC. To my surprise, I saw "FAILED: 1" indicated. Upon further investigation, it was apparently an autothrottle failure, with the FMC indicating "AT INOP." However, the autothrottle remained engaged and continued to operate normally. I then tried flipping the AT switch off and on, and changing vertical modes, but it just behaved as if there was no failure.

After landing normally, I rebooted my PC and started another flight. This time I programed an AT failure to occur after 30 seconds. When nothing happened, I re-checked the FMC to find, once again, that it thought the failure had occured. As before, the AT remained engaged and behaved normally.

Both flights involved the 800WL model, the latter using the PMDG house livery with all default options. Is there any known issue with autothrottle failures?