From Mika's comment

"This cannot be reproduced *in the old Pages app*.
—while of course it happens in the new Pages app. => https://documentation.magnolia-cms.com/display/DOCS62/Blossom+module+release+notes

Version 3.4 states:

[Blossom] has been migrated to the 6 UI framework (non-compatibility), but old-style compatibility dialogs can still be registered.
The module provides support for simple dialogs configured by code. However, there are still certain objects that the old BlossomFormDialogPresenter used to make injectable, mostly Vaadin 7 items, JCR Node and node path. For the time being, please keep using the Pages compatibility app for these use cases (emphasis mine). Magnolia 6 UI framework does not have configurable form-dialog presenters, nor Vaadin 7 items.

Now I acknowledge with version 3.4.1 out, it may seem that this no longer applies, but it does. We'll make the up-to-date status more explicit in that regard."