Headcanon breaking?
DA:I is the story of the Inquisition, not the story of Hawke. Hawke's affiliation with the Inquisition is temporary and potentially fatal. If Isabella knows about Hawke's involvement with the Inquisition she is either happy, indifferent, or upset about it but she's still there; her reasons are her own. Maybe she saw the mess at Kirkwall and thought to help stopping the madness of the Breach, or maybe she's in it for the high price tag she negotiated for her skills? Maybe she followed after Hawke? I believe the multiplayer, time-wise, occurs somewhere after the Western Approach is unlocked (One of the announcers of the Multiplayer is the captain of a Griffin Wing Keep.). . .could be before, during, or after the situation at Adamant Fortress. Thus, Isabella could have joined, been hired, or been recruited by the Inquisition after the Fade incident. Hawke has either died or left.
Regardless: Isabella probably has little to nothing to say about Hawke or Kirkwall in general to the other Agents of the Inquisition, other than 'I was there'. They don't need to know the details; Kolgrim and the others don't need to know her life's story if she has no interest in telling it to a complete group of strangers. Some people are more open about their pasts and their present motives than other people are. It's kept vague or an avoided issue due to the relatively stable state of Isabella (alive regardless) barring Romance and Betrayal.
If anything, this gives the player a chance to see another side of Isabella in the form of new dialogue and interactions with different characters than those witnessed in Dragon Age: II. She's had a colorful past, it's not as though her entire life revolves around Kirkwall and Hawke. Maybe she's suspicious of the Katari due to his being a Qunari, which he then corrects by stating he doesn't follow the Qun. Perhaps she finds the Alchemist amusing while most people either pity her or just outright find her annoying. Maybe we'll hear more about her past or the (made vague) events that transpired between the end of DA:II and the present? Maybe she was out sailing, maybe she was evading the Qunari in Nevarra or- who knows? Give it a chance, you may be pleasantly surprised.
. . .Now, it'd be MUCH worse if they came up with some sort of 'Default' Isabella for the Multiplayer that insists certain events happened the way they did.