Spline cancel the mirror when click in Solo

There is a Mirror activated in the spline Im working on
When I press SOLO, the Mirror is canceled although the button still “on” so I have to press Twice.

When I cancel the solo it goes back to disply the mirrored shape.

I’m not sure I understand your issue.
What is your selection exactly before solo? (just one or both mesh selected)

Here: https://youtu.be/N6HxheVYL8Q

Newly created object are always shown by solo (otherwise it gets confusing when you add something and nothing happens).
And repeated/mirrored nodes are considered new objects.

Thank you for explain the reasoning behind

For me as a end user, in this case is about see the object.
When added a modifier such as a mirror now and later on a twist, bend etc, it would make more sense to see the end result.

As for now in your plans, I would then turn off the mirror button (darken it) bc it stays “on” just the button.