Secret empty on PR, but ok on branch and after merge

I have a strange issue with secrets and pull requests. The secret is available on a branch, but not on a pull request from the same branch (secret empty). After having merged the pull request the secret is available again on the target branch. I have read Problems with Secrets but the problem remains.

Using latest docker in a docker pipeline.

kind: pipeline
type: docker
name: default

- name: hello
  image: debian:buster
  - echo $${SECRET}
          from_secret: secret

On the branch:

+ echo ${SECRET}

On the PR:

+ echo ${SECRET}

After the merge:

+ echo ${SECRET}

I have exactly the same behavior with $SECRET instead of $${SECRET} in .drone.yml
The secret has been created at repo level through the web-ui.

Just found Empty secret when build for Pull Request

Will install drone cli and try to configure the secret for PR.

In fact the doc is pretty clear about secrets and pull requests. Sorry for the noise !