Tooltip doesn't appear above widgets when the chart is placed inside NestedScrollView

I have a screen where there is a fixed Appbar followed by a SliverAppBar below containing the Fl Chart. The tooltip fails to draw over the Fixed Appbar when required.
However, when I place the Fl Chart just below the Fixed AppBar without a NestedScrollView the tooltip draws over it without any trouble.
Can you please explain this behavior?
How does the tooltip draw and how can I achieve my desired behaviour.
I need to have the Chart inside the SliverAppBar as a requirement.
Please Help!

I am using 0.11.1 version.

Reproduce the report using FL Chart 0.11.1 with a fixed AppBar, a SliverAppBar, and a NestedScrollView, then compare it with the layout without NestedScrollView. The issue names no source files or tests, so begin with that minimal widget setup. Done means the tooltip behavior is explained and the required chart layout has a supported solution or a clearly documented limitation.

Written by the indexing model from the issue text.