@alexsavin: the most recent release (v1.1.1, visible in the sidebar) does not have this problem. What you should do is close Sia, download the new version, delete your 'renter/hostdb.json', and then run the new version. After a few minutes you should see the host count start to climb, until it reaches ~70 or so.
@Taek UPDATE! I'm tentatively optimistic that the problem has been cured (for the time being). I deactivated IPv6 in the network settings and it seems to have stopped the auto announcements. It's been about 36 hours without one occurring and I've received a couple of contracts during this time and more storage has been allocated. Deactivating IPv6 doesn't seem to have affected any other programmes, but this may cause a conflict further down the line when IPv6 adoption becomes more commonplace. Feel free to request further logs in future if it will help you counteract this problem. The refund offer is much appreciated. Good to know you're a fair person. I'm sure this is only one less thing in a list of many for you to worry about, but if there's anything we've both learnt it's to keep persevering!