IRS2Go App Shows "Letter Requesting More Information" After Return Accepted - Filed Same Info As Previous Years

My return was accepted by the IRS but when I check the irs2go app it's showing a different message than I expected. Instead of the usual "approved" status I'm used to seeing, it says "We received your return and sent you a letter requesting more information. Please respond by following the instructions in the letter. If you don't respond, your refund amount could be changed. It may take 2-3 weeks for you to receive the letter." It also references "Tax Topic 152, Refund Information" at the bottom of the screen. This is frustrating because I filed exactly the same as previous years - same dependents, same employer for 8+ years, nothing has changed in my situation. I haven't received any letter yet, and now I'm anxiously waiting for something to show up in my mailbox. Anyone know what this means or experiencing the same thing? Should I be worried? Does anyone have experience with how quickly the letter actually arrives? The app says 2-3 weeks but that seems like forever when you're expecting a refund.