Here is a short description of the problem. For clarification, I have created two fiddles in which you can test the behaviour. If there are no suggestions for a solution, I will create a bug report.

Tabulator affected Version: >= 5.5.0
Previous Version (without issues): 5.4.4

Both problems are specific to version 5.5.0 and higher -> https://jsfiddle.net/syh7kjrm/9/
var table = new Tabulator("#example-table", { height:"200px", data:tabledata2, layout:"fitDataStretch",

The problems do not occur in version 5.4.4. -> https://jsfiddle.net/5o09y8nd/20/
These problems are also present in the latest version 6.2.1

Issue 1: Vertical Scrolling in Grouped Views

Description
In version 5.5.0, grouped views do not allow for vertical scrolling without manually setting a large renderVerticalBuffer. It appears that the automatic calculation of the buffer is not functioning correctly for grouped views.

Issue 2: Group Expansion Outside Initial Viewport

Description
When expanding groupings that are outside the initial visible section of the window, the grouping expands but the view jumps to the beginning of the table instead of keeping the expanded section in the visible area.

I expect the same behaviour as up to version 5.4.4. I think that a change/optimisation of the render is responsible for the behaviour.