Update: Received the dreaded "NOT STRAIGHTFORWARD" email today (Business Day 49.) Not sure what next step should be - sit and wait (again)?
I got the same yesterday. A little background, but our MP's office had actually just gotten us a status update on Thursday of last week where they said that my case had no flags indicating it would take longer than 12 weeks to process and that this week was my 12th week. Then I got the email yesterday (business day 58 from app submission).
My partner contacted the woman we've been working with in our MP's office to try to understand what would have changed within 2 workings day to take us from no flags to our case being suddenly too complicated to process as usual, and she didn't seem too concerned about it.
She said that the email is computer-generated if the end of the standard processing time is approaching, but that it doesn't mean much. (Other than yes, they're buying time.) She seemed to think we wouldn't be too far off the usual processing time. She's going to call to find out more, but last time it took her 2 weeks to get in touch with them, so who knows? I'm hoping she'll let me know if there are newly-identified problems that we can address.
On the other hand I know some other people here have gotten the email sooner within their standard processing period and have been waiting awhile since then to hear anything further. Seems like the rules are there are no rules.
I know this isn't much help, but I wanted to share the info I have.