Entity Embed module does support contact forms

I'm having a problem where Entity Embed does not recognize the forms on my Drupal site. Here are the steps I took:

1) Installed the Entity Embed module as well as the Embed module that was noted in the Entity Embed instructions as also being necessary in Drupal 8.

2) Enabled Other / Embed and Filters / Entity Embed.

3) Went to Configuration -> Text formats and editors. In my Full HTML setup I brought the "E" icon down to my toolbar, and also enabled Display embedded entities, then saved. That setup gives the default "Node" embed.

4) Went to Configuration -> Content Authoring area and defined additional embed buttons: one for Custom Blocks and one for Forms. I brought both of those "E" icon buttons down to my toolbar.

5) Here are the settings I used for the Forms button:
Embed type: Entity
Entity type: Contact message
Bundles: all of my available forms were shown but I did not select any of them. As per the screen, "If none are selected, all are allowed".
Allowed Entity Embed Display plugins: Again nothing was selected here, because as above "If none are selected, all are allowed".

6) I tested by creating a new article. I found that both the Node and Custom Blocks do work. After pressing either of their respective buttons, when I start typing the name of an item the module does start recognizing that name and will present me with a list of choices available on my site.

7) Forms, however, will not work. When I start typing newsletter (to match the form "Newsletter Signup" I created), or personal or website (to match the default existing forms), nothing is recognized.

And yet, when I defined my Forms embed "E" icon, it happily not only allowed me to choose Contact message as an entity type, but also under the heading Bundles showed me the exact names of the three forms on my system.

BTW, instead of leaving the three form names unselected as described above, I also tried enabling them all as well and resaving. No change.

Obviously the installation of my Entity Embed module is working, as I can embed nodes and custom blocks. So why does it not allow me to embed forms, even though during the configuration of the "text editor embed buttons" it recognized and showed me the names of the three forms on my system (two default and one defined by me) in the Bundles section?