Older builds allowed to wait for a throttled stage which a newer build is in

The docs suggest that marking a stage with "concurrency: 1" will allow only the latest build to run, and that if older builds reach such a section, it will stop executing:

"The stage command lets you mark certain sections of a build as being constrained by limited concurrency.
Newer builds are always given priority when entering such a throttled stage; older builds will simply exit early if they are preëmpted."

I instead observed an older build simply waiting until a newer build had finished, and then continuing to run the concurrency: 1 section.
This is a problem for us this is our deploy stage, and we wish to only deploy the latest code.