If it's a fairly short prison sentence the lender might be prepared to accept a break in payments but they might also require some evidence showing that the person will be able to return to their job when they leave prison. (For the vast majority of people sent to prison, this won't be possible. Even after a very short sentence, most released prisoners are unemployed for some considerable time).
Additionally, the mortgage lender might be concerned about the mortgage remaining in place while the property (which, of course, is their security) is uninsured. This could simply occur because the convicted person can't make the insurance payments but also because many insurance policies aren't valid if the house is left unoccupied for a lengthy period. (That 'lengthy' period isn't necessarily that long. Some insurance policies state that the property mustn't be left unoccupied for more than a month).
Chris
(PS: I've had some limited involvement with the prison education service. Prisoners have told me that when they've lost their homes, whether rented or mortgaged, they've often lost all their furniture and most of their other belongings as well. This is because probation services can only offer very limited storage facilities for prisoners' belongings. This basically means that their clothes might be stored for them. If the prisoner has nobody outside prison to organise the sale or storage of their furniture, books, TV, etc, the whole lot is simply taken to a refuse dump).
Chris