OK I tested it and for me on Mac with LR Classic 13.2 it does the same in LIB and DEV: CMD S only most selected file - context menu all selected files.
The full behavior of Save Metadata To File with multiple photos selected hasn't changed at least since LR 3, and Adobe considers this "as designed":
- When LR is in Library Grid view (multiple thumbnails showing in the main panel), then invoking Save Metadata To File from the menu bar, the shortcut Ctrl/Cmd S, and the right-click context menu all operate on all the selected photos.
- When in Library or Develop Loupe view (a single image showing in the main panel), then invoking Save Metadata To File from the menu bar or via the shortcut Ctrl/Cmd S only operates on the most-selected photo. But invoking it via the context menu operates on all the selected photos.
- With multiple displays, the behavior is determined by the mode of the main display, even if Grid view is shown on the secondary display.
In general, the behavior of LR with multiple selected photos is highly inconsistent, with different commands behaving differently:
https://community.adobe.com/t5/lightroom-classic-ideas/p-consistent-behavior-when-multiple-photos-selected-in-loupe/idi-p/12360578
Even experts posting here can't remember all the rules.
I tested Save Metadata To File just now on LR 13.2 on both Mac and Windows, and it still behaved the same way.