Picard don't keeps the capitalization of instruments as credited but uses lower cases.
E.g. Recording b1f8a0ef-4402-49dc-8a3d-b909331af6f9: There is a relationship for piano as "Klavier".
In Picard the tag is "Performer [klavier]" (or "performer:klavier" in edit window), i.e. with a lower-case "k".
The capitalization of the credit should be kept, i.e. it should be "Interpret [Klavier]" (here in case of a German interface, but the interface language should not be relevant).
It's not a big problem and depending on the language it might be less relevant. However, I see no reason to use lower case always. The capitalization of the data should be kept.
It will also be written wrong to the file, e.g. "performer=Hubert Giesen (klavier)" in case of Vorbis comment/FLAC.
P.S.: I've tested it with v2.3.2 (EN) and 2.4.4. (DE, portable).