I cannot swear to it for all Travel Insurance companies, but I think it is the norm for them to refuse cover for conditions where you are still awaiting a diagnosis. This appears to be true, even for the companies that specialise in people with pre-existing medical conditions.
You need to remember that TI firms are just out to make money: they want to make a profit for the continued health of their company, and for the income of their shareholders. So, if there's no diagnosis, as yet, they don't know what they may be covering you for, so it becomes too big a financial risk for them.
I would suggest having a single trip insurance for an immediate holiday, then buying the annual multi-trip policy once you have your diagnosis out of the way.
How you would feel - healthwise - about travelling with the undiagnosed condition not covered, obviously depends on your condition and how well you feel yourself ... but also where you are going on holiday. I would talk to your consultant about that, if possible: find out if they would be happy for you to travel without insurance for the condition.
Another option maybe to find a TI Consultant, or general Insurance Consultant who will be able to take your details and search through lots of TI companies.
Sorry to be negative, but I have a wacky diagnosis that does not really exist, and the amount of sheer rubbish that so-called TI 'medical advisors' have said to me is beyond belief. I am considering making my own recordings of their conversations, and maybe writing something about it all.
Not that I'm bitter and twisted or anything.
Whoever you go with, tell them everything: even things you are not bothered about having covered need to be declared, and then noted as 'not covered' by them. Often they have clauses that invalidate all cover if there's something you have not told them.
Also, always read all the small print in the policy - everything.