DateTimePatternGenerator::AppendItemNamesSink::put() doesn't handle alias types even though alias types may be present for display names. This leads to throwing a U_RESOURCE_TYPE_MISMATCH error when calling ResourceValue::getTable(), which then leads to exiting DateTimePatternGenerator::AppendItemNamesSink::put() early. See for example "data/locales/root.txt" for when alias types can be present.
This can lead to similar issues as in ICU-20739, for example "minute" may suddenly be described as "F12" or "day-periods" as "F10".
Here's a simple patch I've used to avoid the U_RESOURCE_TYPE_MISMATCH error, the actual alias support still needs to be added, though.