Only one concurrent build allowed after moving to Xcode 9.0

Hello,
We have recently started moving from Xcode 8.3.3 to Xcode 9.0. We are using the free plan for open source iOS projects (https://circleci.com/gh/wireapp). We used to have several concurrent builds running at the same time, but after moving to the newest image it seems there is only 1 build running at a time.
Is it a new limitation for the open-source plan?

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