You have every flag for interacting with Demon Syri after the fight with her, but NOT the flag that is set for actually encountering her. That flag has been there in the AS3 code for at least 3 years, and it's been there since before the first public release of the JS port so this isn't a case of it being a new flag that was only added later on.
At this point I can only assume there was a version where some of the code added for the syri hellhound stuff was mistakenly clearing that core flag rather than testing it was actually set. However, because we had a bunch of other flags downstream of that initial "have you encountered her at all" flag, we can infer if it should have been set from the more specific flags that happen in her post-combat interactions.