2. Select column B and drag mouse towards C, both columns will be selected which is correct behavior
3. Now just scroll down 2-3 rows using vertical scroll bar
4. Repeat step 2, this time it does not select B & C, instead it selects B,C,D,E.
5. Seems a bug, not an expected behavior.
Observation that might help you in fixing it:
1. if you move scroll bar in such a way so that no merged cell is visible it works well, e.g. scroll down till 20th row becomes first visible row on screen and now repeat step 2, it will work
2. if scroll position is on top then behavior is correct as well
3. Same issue exists in case of multiple row selection with merged column and scroll position.