After the oven has been on “really hot” and you turn it off, the clock display goes blank.
If you open the oven door for a few minutes and let it cool down, the clock then displays “STOP”.
It can be reset from this point in the same way as you do when the power goes off.
It doesn’t appear to make any difference to the cooking, however.
There will be a thermal cut out either on the top of the oven or down the back which is either defective or needs moving – when the oven gets too hot it cuts out the power to the clock, once the oven door is opened and the temperature goes down it resets itself until the oven overheats again.