The thread used to address this issue is Adjusting Capture Date/Time
Ive used this method many times before because though my cameras are all synced, there can be 1 or two other photographers working with me and I can count on there camera's clocks never being even close to the real time. Most times if I remember while on location I'll have us take shots of each other simultaneously as a time base to correct all the other images.
Problem is, its no longer working?! I'll select all the images that need adjustment and "Most Select" the datum image, set "Adjust to a specified date and time" I notice at the top of the action window, where your screen shot says "Modify the capture time stored in the selected photos" mine still says its changing this one "Most Selected" Images. and when it runs thats all thats done.
I double checked and there are 418 images selected and the images shown in the dialog box is the Most Selected.
Am I just getting old and missing something?
That's strange. Try resetting LR's preferences: https://www.lightroomqueen.com/articles-page/how-do-i-reset-lightrooms-preferences/ . LR not infrequently soils its preferences file, and resetting often fixes things when the user interface goes wonky.