The input once entered as 20 : 00 and clicking OK resolves to 20 : 0 when you come back in; whilst it appears correct, it just doesn’t look right. I tested it and if you set it to 20 : 01 when clicking OK that resolves to 20 : 1, so it seems it removes insignificant zeros.

You could set it to 20 : 10 which remains as 20 : 10 when resolved.