sharepoint - Required field in custom content type not required in custom list instance -

i have issue custom sharepoint 2013 solution. among other components, consists of:

• a feature several custom fields different types, of taxonomy fields,
• a feature 3 custom content types using different sets of custom fields partially different configuration, e.g. whether required or not,
• a feature custom document library template , instance using 2 of custom content types , default picture library programmatically customized when feature activated, e.g. assigned third custom content type.

when deploying solution , activating feature, set correctly except single taxonomy field 1 of doc lib's content types. defined required in both content types indeed not show required default content type whereas works fine other. , not matter of 2 custom content types defined first (= default) in list template's schema.xml, issue occurs same taxonomy field in doc lib's default content type. when use built-in document content type default, field required both custom content types. however, using document content type not option.

if think behavior not strange enough: when list content type's field set required through sharepoint ui, becomes optional again when list column updated, e.g. display name changed.

any ideas?