This seems like a bug in the bitucket-pipelines.yml validator, when a step's property children are not indented, the validator doesn't report an error but the pipeline will fail to run, for example:
definitions:
steps:
- step: &step-1
name: This is valid indention
- step: &step-2
name: This is invalid indention
step-2 above will cause the pipeline to fail, but the validator doesn't complain.