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:
11.2 and previous versions: "AX" (correct)
11.3.1: "< mixed >X" (incorrect)
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.