@Louis, I have confirmed that the behaviour is caused by the copy linked feature as @Marius pointed out.
So the problem as stated is not technically a bug.
However I have identified the cause of the confusion and why we didn’t spot this sooner.
When using the top toolbar - the toggle state is not shown:
Selected state using editor toolbar:
Selected state using top toolbar:
Effectively there is no way to know whether the copy linked feature is on or off when using the top toolbar.
Also, @Louis, when you have a moment, please can you confirm these questions, as I am still trying to understand why this is toggled for them in the first place: