bamboo.planRepository.previousRevision is only populated in the first build when using commit isolation in SVN

When commit isolation is enabled in a SVN linked repository, bamboo.planRepository.previousRevision is only populated in the first build in the series of build for the list of commits.

Steps to reproduce

1. Remove the trigger from the plan
2. Making multiple commits to the repo
3. Re-add the trigger and let Bamboo trigger multiple builds

Expected output

All resulted builds should get the same variables

Actual output

bamboo.planRepository.previousRevision is only present in the first build in the series

${bamboo.planRepository.previousRevision}: bad substitution