Using Views Bulk Operations with custom view type (GBIF 2.0.x)

Problem/Motivation

I wrote a module which provides integration with GBIF (Global Biodiversity Information Facility). It contains a submodule to directly display remote GBIF data with Views. It is possible to couple it with VBO to locally process GBIF occurrences (for instance, migrate data to Drupal, with some alterations).

This worked just fine in Drupal 7 (I only had to use a "fake" entity with a table which remained always empty, in order to satisfy VBO). However, after porting the module to Drupal 9, I am unable to replicate this behaviour.

I followed this piece of documentation on view providers API, hence this event subscriber and this entity getter.
You can also take a look my implementation of hook_views_data(). Although VBO will not complain, two weird things happen when I try to use actions on this type of view:

1) There is a frame ("Selected 1 item in this view") which does not appear on a node-based view, plus, it detects no item even though one check box is active:

2) The confirmation page also displays "No items" instead of listing the selected entities, and my action is not fired.

There is an entity integration using the External Entities module. However, I gave it a try with $event->setEntityTypeIds(['node']); in the event subscriber while returning a node in the entity getter, and observed the same behaviour. Please not there is is no `_entity` key in rows (see the query plugin). I tried to add one there, to no avail.

Steps to reproduce

Install the GBIF (gbif2_views, which should depend on gbif2 and gbif2_entity) and Views Bulk Operation modules. External Entities will be required by Composer, otherwise download it manually. Check that a GBIF occurrence external entity (admin/structure/external-entity-types) is created (based on this piece of configuration). Build a view based on "GBIF occurrences" and add an action.

Could you please help me find what I have been missing in my integration with Views and VBO?