My ex did the same a year before we divorced, £300K, mortgage interest only, DJ factored it in as negative equity, he also said he has a large loan from his sister but did not provide proof but it again was factored in.
Sorry but, its all down to the postcode lottery and the DJ.
The day after the CO he paid off his mortgage in full!
It will need to be included in your financial arrangments and your Consent Order, so that its value is taken into account. You won''t have a claim to it after the finances are settled, but by then you won''t need to as you will have had compensation for any equity he has in it.