Can't Pre-Assign Breakout Rooms

I've used breakout rooms numerous times, and the majority of the time I use them I pre-assign the participants before the meeting. However, one specific meeting that I just scheduled won't let me assign participants. While in the breakout rooms settings, the "assign participants" option is there but it is greyed out. Almost as if the participants aren't showing up in the Teams calendar invite. However, I can see that these users have accepted the invitation. These users are all within my organization, too. Is this another Teams update/glitch?