Can't stack images anymore

I am having this same problem ~ suddenly all my stacks are expanded, but with no little bars on the first or last shot to facilitate collapsing. I have restarted LrC several times; in addition I have selected the photos I wish to stack, R-click on them > Stacking > Group into Stack ~ but nothing happens.
I am on a Mac Studio, running Sequoia 15.0, LrC Vs. 14.0.1. My LrC library is on my Synology NAS, as are all my photo files. I have used LrC for many years, but this issue began within the last month. I can't verify it was at the same time as my last update on 11/04/24 ~ but it seems a bit suspicious.
Thanks for any suggestions.

I can't see this on my Windows machine.  

In a first step you should  try to reset the Lightroom preferences.

https://www.lightroomqueen.com/how-do-i-reset-lightrooms-preferences/

It's recommended to backup your preferences before you reset the preferences to the default settings:

https://helpx.adobe.com/lightroom-classic/kb/preference-file-and-other-file-locations.html

Unfortunately you haven't given us any information about your system. Is your system up-to-date? Do you have installed all recent updates and patches for the operating system and the device drivers, especially the graphic driver. If you have a NVidia graphic card installed, make sure that you are using the recent Studio version of the driver, not the Game Ready version.

Please provide a copy of your 'System Info'. This can be get from the LrC Help > System Info menu item. There's a copy button in the System Info dialog. Press this button and paste the info into your next forum post.