Of your 2 Olive Ash pieces this is my favorite, the 'Olive' is dark and dramatic and there is even some ripple around the knot.
Yes I had a bord that made 3 such platters, Which I'd rough turned. The first one I finished warped which made me wait to turn the other 2.
When I was eventually happy they had settled, I refinished the first and made the other 2 to match.
It's the temperature/Humidity change affecting the moisture content of the water. With central heating, you're right, it's normally a case of further drying out. With larger pieces it's normally a good idea to bring them into the home for a week or so (or longer) before finisheg the turning to let the timber stabilise.