In my most recent game I chose the Norse culture. The Langskip EU worked as expected, and I eventually shifted into the Mughal culture. The Langskip EU bonus persistend until I saved and reloaded the game. At this point, I received a notification telling me that one of my units was about to be lost at sea, which wasn't true before I saved the game. The Langskip was still the most advanced naval transport unit I had researched. Note that the additional movement of the Langskip remained available, only the 'High Seas' bonus of the EU seemed to no longer be active. 

2 most recent saves, both including this error (unfortunately not autosaves)

1) Mughals Turn 221.ctr

2) Mughals Turn 223.ctr

Diagnostics taken around the time of the save: Diagnostics (2021'08'22 @1137'45''885''').html

Player log:Player.log

'Lost at sea' warning present immediately upon loaded save: 

Evidence of having taken Norse culture previously (Stormborn trait): 

Evidence of Langskip bonus movement still remaining after load: