Wow, Thomas, I didn't know you could use Extend Edit to slip a clip! That's cool... but yes, I can confirm the same behaviour on both Windows and Mac.
Loss of sync is not registered when using the Extend Edit function to slip a clip, thus no red 'out of sync' indicator is displayed after sync loss using this method. Manually pushing such an un-synced clip even further out of sync (or back into sync) will result in the expected 'out of sync' indicator, being displayed but the indicator will be incorrect, registering only the additional sync loss that happens ie after the original loss of sync.
Good find.