In the Expert workspace, select Timeline clips involved so that they are all selected on the Timeline at one time
Then go to Timeline Menu/Apply Default Transition. It is done.
Two supplemental considerations....
1. Selecting more than 1 clip at a time
a. As you hold down the Shift key, click on each of the files to be involved
b. With the mouse cursor, draw a rectangle around adjacent clips to select them
c. If only the clips involved on the Timeline, then Edit Menu/Select All
2. To set the default transition...
a. Go to the Expert workspace Transitions Tab at bottom of workspace...open to the transitions display...find your transition, and just right click it to bring up the pop up "Set Selected As Default Transition" which you click on.
Have you updated 12 to 12.1 yet using an opened project's Help Menu/Updates?
Please consider and let us know the outcome as your time permits.