Just wanted to share that I still have both Ruby Oven and Sapphire Oven in my bakery's market. I also checked my other two accounts and I can still see both ovens in there too, same prices also.
Here's a screenshot from my main bakery:
Edit: In my main bakery the Sapphire Oven price was 25 gems, in my other two accounts it was 15 and 35. So apparently after the timer is over the oven goes back to the highest price, which is exactly what I thought would happen to be honest 😅