In following scenario we are receiving EditedDateTime value even though message has not been edited.
Subscribe Ms-Teams subscription https://graph.microsoft.com/v1.0/subscriptions with chat resource. e.g https://docs.microsoft.com/en-us/graph/api/subscription-post-subscriptions?view=graph-rest-1.0&tabs=http
Send message from one of the Team or Chat Item. {Do it from https://teams.microsoft.com/_#/ browser}
Receive the notification with createdDateTime value but lastEditedDateTime and deletedDateTime are null respectively.
Delete the message from {https://teams.microsoft.com/_#/ browser}
Receive the notification with createdDateTime and deletedDateTime with value but lastEditedDateTime value is null.
Undo delete action from {https://teams.microsoft.com/_#/ browser}
Receive the notification with createdDateTime and lastEditedDateTime with value but deletedDateTime value is null.

Looks not correct because original message was not edited any time.
When I called a https://graph.microsoft.com/beta/teams/<TeamID>/channels/<ChannelId>/messages?$top=1 from Postman
I got the correct details. Receive the data createdDateTime with value but lastEditedDateTime and deletedDateTime are null respectively.
In real-time notification, we are getting incorrect data. I have attached snapshot for more information.
The cause is we are not able to understand whether the message was actually edited or undo deleted.