FWIW this is not a new regression, we noticed this ages ago, and have been using stash/unstash for ~6 months.
I hadn't realised this wasn't intended behaviour.
Nothing complicated, just a normal project, not a multibranch project, see attached.
Jenkins version 2.30, I updated all plugins yesterday, I can give you specific versions, but here are the pipeline ones:
Pipeline: API - 2.6
Pipeline: Basic Steps - 2.3
Pipeline: Build Step - 2.3
Pipeline: Declarative Agent API - 0.5
Pipeline: Groovy - 2.22
Pipeline: Input Step - 2.3
Pipeline: Job - 2.8
Pipeline: Milestone Step - 1.1
Pipeline: Model API - 0.5
Pipeline: Model Definition - 0.5
Pipeline: Multibranch - 2.9
Pipeline: REST API Plugin - 2.2
Pipeline: SCM Step - 2.2
Pipeline: Shared Groovy Libraries - 2.4
Pipeline: Stage Step - 2.2
Pipeline: Stage View Plugin - 2.2
Pipeline: Step API - 2.5
Pipeline: Supporting APIs - 2.10