Notifications not working for promoted builds plugin Mattermost notificaition as a promotion process action

[twegener-embertec]

Mattermost Notifications are working fine for Post-build Actions for me. However, when I configure Mattermost Notifications as promotion process action (from the promoted builds plugin), and tick all the boxes there, I do not receive notifications when promotions start / finish.
(In jenkins.log, it shows the start / finish notifications for the build, but not the promotion.)

Note that with the HipChat Plugin, notifications for promotions do work.

Mattermost Notifications 2.6.2
Jenkins 2.150.2