I think we need to change some numeric date formats in ja (at least for japanese calendar) to use GGGGG instead of G.
Currently we have in ja:
As a result, in ja-u-ca-japanese, an intervalFormat request for skeleton GGGGGyMd is getting formats that use Gy年M月d日 (in ICU4J, but not ICU4C). This came up with recently added tests in ICU4J.
Note that en more consistenty uses GGGGG for numeric date formats:
So the fix here in "ja" is either to change the generic formats to more consistently use GGGGG for numeric, or to add formats for japanese calendar to specifically do that.
Note that in "ja", changing generic would not affect chinese calendar (which already has its own intervalFormats), and would help buddhist (which has its own availableFormats already preferring GGGGG with numeric data, but also inherits generic intervalFormats).