I'm going to ask a ton of questions and they're more process and efficiency related but they're all about dialing in your new system so not really off topic.
What was your mash efficiency? Brewhouse efficiency? How did they compare to previous iterations of this beer? You didn't use a completely new recipe on a completely new system did you?
It's all about consistency and since you have a new system you have no baseline for efficiency until you get a few brew sessions on it. Your equipment changed so your process will likely need to be adapted to the new equipment especially if you're not boiling as hard with the new system.
Do you typically have 5.5 gals post boil? An extra half gallon could have a big impact on OG.
Do you take gravity readings of first runnings? Sparge?
My guess without more info is you didn't boil as hard as in the past but it's hard to say without a baseline comparison. It's going to take some trial and error to work out your efficiency. Be anal about volumes and record keeping. Software helps. I rely heavily on beersmith.
I'd figure out your efficiency for that first brew session. Consider that your system efficiency number and plan THE SAME RECIPE around that efficiency for your target OG. That will give you a baseline for future recipes.
I know you're not a noob but sometimes it helps to go back to square one and think it through.