Codebuild won't run more than 1 build concurrently

Same question was asked here but the answer does not apply to my case: https://repost.aws/questions/QUxsq-Hv5GSPa_Pth-5ZVckQ/cannot-get-code-build-to-build-more-than-1-concurrent-build

We've got 2 pipelines, each with a Codebuild, making a total of 2 Codebuilds. I've noticed that the Codebuilds spend most of their time in the QUEUING status, sometimes remaining there for as long as 10 minutes. The only explanation I have for this is that each time this happens, there's 1 other Codebuild running. We have not set any limits and are running using the SMALL compute type.

I've read the quote limits and we certainly are not even close to those limits, maybe running the pipelines a total of 5-10 times per day, meaning 5-10 Codebuild invocations per day, maybe 10-20 Codebuild invocations per day on a busy day.

What could be causing this? Please don't say we must have a concurrency limit set somewhere and not know about it because everything was created via CDK and no stack has drift.