Dear Mr Irie, can you point me to the place where you published the results of your discussions with Suburbia users prior to this change? Did it involve a survey at all? Did I miss out on that discussion?
Your fix above works, thank goodness, but for anyone else, this means to get back to the old behaviour you have to pay for the CSS upgrade, which means I won't be using Suburbia any more, as it adds an unnecessary cost.
I think I'll have to reconsider using WordPress if backward compatibility is broken so easily and without me having any control over a key asset, the Theme.
Any decent producer of software makes sure the impact of design changes is considered beforehand, that impacted users (such as me) get some prior warning of the change, and are given an easy option to opt-in or out of this fundamental change.
So far I have proselytised to people about the goodness of WordPress, now that belief is undermined. I was going to roll out the HertsHPS site to other groups around the UK, on the assumption it was stable, and that non-technical users could maintain their site. Your CSS change has undone that intention.
This is not a good outcome and shows your firm in a poor light - at least consider a form of version control for themes - leave Suburbia V1 alone, and launch V2 so users have a choice.
Overall this is not a good outcome at all. Your comment above doesn't clarify why you made the change at all - it isn't back up by any background on the 'discussion' process, nor any survey results on who voted for this change.