I saw the same thing while rebuilding. The only indication, AFAIK, is that the mailbox in question seemed to have its emails disappear until the rebuild was complete.

Jon

macOS 11.7.10, iMac Retina 5K 27-inch, late 2014, 3.5 GHz Intel Core i5, 1 TB fusion drive, 16 GB RAM, Epson SureColor P600, Photoshop CC, Lightroom CC, MS Office 365