Unprofessional/inappropriate questions.Felt like a criminal

My partner and I applied for a mortgage ( 80% LTV). I have over 100k in savings and wished to take a second buy to let property with my partner. Because my partner is in Rented accommodation I was asked questions around Aldermore's concerns that my same gender partner would be living in the property. Where they came up with this information is beyond me. I own 4 properties myself, and it would make more financial sense to have him live with me- but they came this conclusions and wanted this clarified and evidenced before considering.
Then I was informed I had a default in the last 3 years. I have an erroneous default for 15 pounds noted by T mobile. This is a false entry and I have asked for it to be removed. It will take six weeks to resolve. I contacted them to ask that a senior manager take a look at this file. They declined. My mortgage has been refused. I have been in the same job for 25 years. I earn over 100 000 a year net. I own 5 million pounds worth of property. ( anyone in the business knows it makes sense to take out a tax deductible interest only mortgage on buy to let property)
Yet for a 15 pounds erroneous default ( and an impeccable credit rating of 973 on Experian and Equifax- I have been rejected.
I have been accepted by Virgin; HSBC; Barclays and Halifax subsequently with no issues raised and no fuss and better rates!
I complained. No investigation took place. I was not contacted. But a FINAL decision was issued instructing me if I had any further concerns to contact the ombudsman( which I have)
Very unprofessional. Appears very parochial.