No, you are wrong in a number of ways. This is to my knowledge never how it worked in previous versions, iirc the code from pre 1.5.0 would only ever assign a captain to any formation if the player got the screen where they can choose. Please provide a source that proves something other than the following for 1.5.0.
Just debugged with dnSpy to be sure and here it is. (1.5.0 dlls)
This is the code responsible for captain assignment
Unless the player choose formations (the screen at the start of battles when in army), the above code should be used.
The captain for a given formation is chosen by checking the units in the formation. The first alive hero unit found is chosen. There is only 1 captain for a formation, if it dies the formation tries to find another alive hero in the formation to be the next captain, if it can't find one no captain is assigned.
The player is not in a formation and so will never be chosen as captain (unless it got to choose a formation at the start of the battle).
The player can still order a formation that has a companion as captain (no need to F6).