Ver. 1.3.2
If you select frames by pressing the SHIFT key and clicking on the frame at the other end, Aseprite doesn’t apply the changes, at least not to the frame durations.
To avoid this, use the mouse-dragging method to select all the frames whose duration you want to change.
In this video, you can see that I select the frames using the SHIFT key, then change their duration. Afterward, I select a single frame, and you can see that its duration hasn’t been changed.