Will Probate ever require an heir to obtain a new mortgage?

[Samantha Cash]

Hoping some folks with probate experience can help. 

My partner's father passed away with out a will and a small amount left on the mortgage and for various reasons we would like to keep the existing mortgage. From our research it seems like mortgage companies generally allow an heir to take over payments, however our lawyer has said there is a small chance they won't allow it. Has anyone seen this and if so what were the circumstances? I want to get an idea of how common it is before I dive to deep into looking at other finance options.

Thank you.