Steps to reproduce the issue

• Install another media-manager (in our case just install JCE, it comes with a cool media manager)
• Create a content-plugin to add fields to the standard joomla articles (we use repeatable subforms, so we have to use a custom content-plugin)
• Insert a media-field as field inside the subform
• It shows the standard (not so powerful) Joomla-Media manager, where as in other areas (modules...) the media-fields media-manager is replaced with the JCE manager

Expected result

Media fields inside content plugins show the 3rd-party media-manager

Actual result

The standard joomla media-manager shows up

System information (as much as possible)

• Joomla 3.9.5 clean install
• JCE as primary editor

Additional comments

I wrote it as a bug-report, since i think that it is a Joomla-related issue, since in modules ecc. the 3rd-party media-manager shows up, only in content-plugin fields we get the not-so-cool joomla media manager.