As usual you are giving them way to much credit for have a brain when it comes to timeline and hiding information. They don't think about that at all, because they don't think they have to, in their opinion they are entitled to everything life throws at them, have no responsibilities towards anyone else and no need whatsoever to hide their tracks. She didn't have to reveal she was chatting from Brussels and could have easily pretended to be at the HMN. The way her vlogs are structured now she has ample opportunity to go wherever she wants to go without a problem for her timeline, which is screwed as it is and people are used to it. She's using the same playbook as MPK, but with better preparations and safety nets in place.
What I've heard is that her trip to Brussels was business related, talking business with the not so elusive N. What I made of that information, based on how things usually go down at her HMN: she announced the arrival of the not so elusive N with family in a month or so, around Halloween, in public on her vlog, when that plan was not confirmed at all just yet. Nic and Marie did not appreciate this as they wanted their stay to be private and not to be filmed. They cancelled the plans. SJ however needs them to make an appearance somehow, hence the abrupt trip to their home in Brussels to "talk things through" in person. A trip with further benefits for SJ, because Brussels can't be far from whgere her boyfriend is "rehearsing his musical".