Hello,
When you create a tailor entries field in taglistmode, relation is not saved.
If I change the displayMode to “relation” it works, so It’s not a problem with the configuration.
Here is the yaml for that fields :
categories:
label: Catégories
type: entries
source: Blog\Categories
displayMode: taglist
Best regards,
renick
#2
Similar issues for me. When I save on a static page, it wipes the content of the “taglist” 
Yaml config for the field:
classes:
type: taglist
mode: string
separator: space
My issue got resolved by setting customTags: true and useKey to false
Its not the same issue, I talk about relation displayed as taglist, not standard taglist form field