The Metadata panel's Caption field doesn't correctly display the results of appending text to the captions of multiple selected photos. It works correctly in LR 11.2 and earlier but not in LR 11.3.1, and it appears to be another symptom of the change in LR 11.3 to cache metadata in <catalog> Helper.lrdarta.
Tested on LR 11.2 and 11.3.1, Mac OS 12.3.1. Steps to reproduce (also see the screen recordings):
1. Make a new catalog and uncheck Catalog Settings > Metadata > Automatically Write Changes Into XMP.
2. Import three photos with no caption in their metadata.
3. Set the captions of the photos to "A", "B", and "C" respectively.
4. Select all three photos.
5. In the Metadata panel, with Selected Photos highlighted, observe that Caption displays "< mixed >".
6. Set the input cursor after the ">" and type "X" and then return. Observe that Caption now displays "< mixed >X".
7. Select just the first photo. Observe that the Caption field now displays:
Make similar observations of the second and third photos ("BX" versus "< mixed >B", "CX" versus < mixed > C".
8. Restart LR and observe that the Caption fields haven't changed.
9. Export the photos back into the catalog and observe that Caption fields of the exported photos have their correct values. This demonstrates that the captions of the originals were correctly recorded in the catalog, just not displayed in the Metadata panel correctly.
10. Exit LR, delete <catalog>Helper.lrdata, restart LR. Observe that the Caption fields are now correct in 11.3.1.
Updates to the Adobe Photography Products were released on June 13. The update contains a fix for this issue.Please download and the latest version of your application via the Adobe Creative Cloud Application or your respective device app store.
If you do not see the update (Mac and Win) you can refresh your Creative Cloud App with the keyboard shortcut [Ctrl/Cmd]+[Alt/Opt]+[ R ].
Note: App store availability can take several days for the update to appear and be available.