Here's an interesting twist. I operated the spoilers on ground using the switch that has been allocated for it as part of my pre-flight checks. When I took off, the aircraft behaved like the spoilers were extended (even though they weren't) as the speed did not exceed 180 knots.
I then abandoned the flight, restarted MSFS and tried again, this time not touching the spoilers before take-off and the aircraft behaved perfectly.
Has anyone reported LNAV issues in the CRJ550 yet? And by LNAV issue, I mean it basically does S-turns to maintain the LNAV profile. It's particularly noticeable when going to another way point, but I find that it never "settles in". I don't think I've had this issue with the CRJ700 variant yet.