This is a TRichViewEdit limitation. When editing too long cells (higher than 30000 or 32000 pixels, I do not remember exactly), the editor sets the cell editor height equal to this limit, enable scrolling in it, and hatches the remaining area.
Any chance to remove this limitation? Because in this case it's just about 2300 lines. We're in 2018 where 8Gb or RAM costs a couple of bucks so I see no reason for such low limit. Worse - the limit is in pixels.