This feels like it happened overnight. Before, I've been able to use one of the selection tools on the spectrogram and then click and drag the side or corner of the selection to resize it. However, I am suddenly today unable to do the resizing, although the square & lasso tools still make the initial selection.
Just updated to the newest version of Audition (18.104.22.168) today, but was happening even before I made that update.
Weirdly, just now a couple minutes later, I now can resize them again. It must have been bugged temporarily, and resolved itself somehow. This may still indicate a bug in the software.