A regression caused by 7a2d04a5c4.
The offending commit made it so selection tag does not imply parameters
update, which also avoids transitive re-evaluation of the curve geometry.
However, the active curve index is stored on a Curve, and curve modifier
stack creates a copy of the curve to hold the evaluation results, which
makes it so evaluated curve object does not intrinsically share the active
spline index.
This change makes it so changes in selection triggers geometry evaluation
on curves, matching the behavior prior to the offending commit.
AN ideal fix would somehow avoid such geometry re-evaluation, but it would
be a bigger change, not suitable for possible corrective release.
Potential candidate for 4.1.1.
Pull Request: https://projects.blender.org/blender/blender/pulls/119918
d3504a6bab