Facebook is so strict! that’s enough to get filtered as spam, and the dm never get delivered
How fast did you send those messages ,most likely you didn’t do a delay between the DMs you have sent
you better try with another account send one DM and see if it well be delivered than you can see some patterns to the reason!
i had been posting in bulk in 2015 that the posts Gets removed Automatically sometimes i will still be able to see the post but when i login from another account can’t find the posts Just like google+ system of filtering spam you post it and you will see it posted but no ne else can see it but you with that account!
Hopefully they will share with you someone else that has better knowledge in this situation can give some sort of help too!!