Bcc Recipients are VISIBLE on Iphone Mail!

I sent an email using outlook with my first email account Bcc to a list of people, and I added my second email address into the Bcc list, just trying to make sure that Bcc works properly. But once I opened that email on my second email account on my iphone, I could see all the 80+ recipients!

(You can see that from the picture below. I expanded the Bcc list and I could see everyone on the list! )

This only happened on iphone, because when I checked that same email on my second email account on my PC and MacBook Air, the recipients are invisible.

I sent the same Bcc email a second time and the same thing happened again.

I repeated the same process and sent another Bcc email again and by the third time, the recipients finally became invisible on iphone and all other devices.

I want to know why did it happen.

I have both my first and second email accounts signed in on outlook, MacBook air, and on iphone. So, I suspected that was the reason why the Bcc became visible, because maybe my first and second email accounts are somehow linked by being signed in on the same device.

But what I don't understand is the reason why that those Bcc recipient were only visible the first two times and then became invisible after that.

(You can see from the second picture that the Bcc is gone)

Random occurrences like that made me feel very unsafe. What if the next time I Bcc a group of people and the iphone just decides that this time, everyone can see each other!

Does anyone have any explanation for that?