I've had this same problem. You're typing in Myriad and want to change the font to Aachen, so you Ctrl Alt Shift M or click to access the Character field. Type aac, see Aachen appear, hit enter and it goes right back to Myriad. I've found that it doesn't happen all the time and it may be only certain fonts it occurs on. I haven't figured out the pattern - Yet!
Has anyone got an update on this? Is there any fix?