I will go for a wiring error
either the cylinder stat isin't moving the valve back to hot water only or the programmer hot water off connection is missing or connected wrongly.
Like the theory, but it would need power on the grey to hold it in the last position.
If I had a Chrystal ball, I would say, one or more rads is being turned off, maybe one of the additional rads in and infrequently used room, so there's no back flow until its turned on.