It's not a bug, you can't change the number of members on the plan after you set it up. So if you set up a 1+1 subscription, you can't change it to 1+2 without cancelling I'm afraid.
See the help page for full details:
What if I want to add another subaccount to my plan? The number of subaccounts (i.e. the number of people you can invite) that you selected during signup cannot be changed. You may however cancel the account – at which point all members of the plan revert to Free level subscriptions if the current billing month lapses. You would then sign up again, selecting the number of subaccounts you want. You may the re-invite the people you want to keep on the plan.
I have the same problem. As a solution cancelling your subscription and waiting for it to revert to free before re-subscribing is a workaround not a solution. I'll try it but Spotify need a proper solution if they want to make Family more appealling.
Thanks everyone for the feedback. Spotify support also contacted me. It turns out you don't have to wait for your subscription to expire. You can go right back into the portal and re-apply for a premium account and then a family plan. In my case I had one family member and I asked for one more (me and 2 family members). It retained my previous familiy member and gave me a slot for the 2nd one I asked for. I was able to invite for that slot.