Menu links losing active trail when not edited in path alias language

We are currently experiencing some strange behavior on one of our sites regarding menu active trails in language-specific menus with untranslatable menu links.

Steps to reproduce

• Set up a Drupal site with two languages (e.g. EN and DE), set EN as default language with URL prefix /en, DE without URL prefix
• Enable all multilingual modules (especially language and content translation) and ensure that path module is enabled
• Use path prefix URL language detection
• Do not allow translations of custom menu link entities and set -Not specified - as default language for them
• Create a language-specific menu (in our case EN as menu language)
• Create an english taxonomy term with a path alias (let's use /term-name for now)
• Ensure that you are visiting the english site version (with /en URL prefix) and add a menu link using /en/term-name as link value
• Check your menu_tree table and search for the row with "route_param_key" having taxonomy_term=[TID] (of course use your term ID for [TID]) - it should exist
• Visit the menu link's page and inspect the menu link - it should have a class added indicating that it is in the active trail
• Switch over to german site version (without /en URL prefix
• When you are on the menu edit page now and click edit on that menu link again, you are automatically send to the edit form without the /en URL prefix (if not, open it up without the prefix). Now just safe the form without any manual changes (NOTE: You are editing a menu link without language for an EN menu, with an english path alias as link... in the german user interface, which should be no problem at all)
• Check your database again... the row with "route_param_key" having taxonomy_term=[TID] (of course use your term ID for [TID]) is gone, but instead of it you now have a row with "url" having base:/en/term-name... but without all the term information the previous row had.
• Visit the menu link's page and inspect the menu link - the class indicating that it is in the active trail is gone, because the menu system cannot identify it in the active trail anymore
• ... even worse: When using the aliased link URL, editing this in the corresponding language without breaking anything etc., the next cache clear will also kill the correct row in the menu_tree table

As this is a rather complex problem, I hope I did not forget any steps to reproduce this behavior.

If anybody needs a workaround until this issue might get fixed:

• Instead of using the aliased term URL with language URL prefix, use /taxonomy/term/[TID] (of course use your term ID for [TID])
• Check your menu_tree table and search for the row with "route_param_key" having taxonomy_term=[TID] (of course use your term ID for [TID]) - it should exist
• Using this URL based on the internal path you should be safe to edit the menu link in any interface language you like