messenger notifications grouping on iOS 12 - stacks not working for all messages

I have a problem with notification grouping on iOS 12 for Messenger (Facebook) on iPhone XS and iPad Pro 11.

On setting called „automatic” when I receive more than 5 messages from a group chat where are two or more people involved, notifications are nicely grouped into a single stack - two group conversations, two stacks etc. so far so good.

BUT few messeges from a single person are not grouped into stack (even when more than 5). In the morning I have for example 53 „notifications” waiting for me - combining hundreds of messeges from dozens of people into 3 stacks from different group conversations - and 50 single notifications about every single message from just one person.

I think that messeges from single person should go into their own stack.

Currently it’s cluttering my Notification Center. Messenger is contributing more than a half of all my notifications as I checked in weekly rapport so you can understand that it’s an issue.

I can set notifications as “always” grouped in settings, but then there is single stack for everything, it’s not a good solution.

I think it should be simply one stack for a conversation, be it from group or a single friend.

Or it’s not working as intended for me only? because on my fiancées phone (iPhone X) it *seems* to work as it I think it should. Is there a solution that I missed? I tried reinstalling but it did not help.