Agree with Devora
I wouldn't see it as a breakdown in relationship. Some (older) children are adopted who love their birth family members very much. Birth parents nearly always love their child and don't want them to be adopted. However they can't parent safely or appropriately, so adoption is necessary. In modern adoption, there is no concept of a child being a blank slate, but an importance placed on recognising that they have another (birth) family and that this family may be very important to them.
Typical contact is one or two letters a year, which may included photographs or may not. I've not heard of letters undermining the adoptive family at all. That's not been my experience anyway.
A smaller proportion of children may have direct contact. It's unusual but when it happens, it's normally again either once or twice a year.
Also, most agencies facilitate a one off meeting between adopters and birth parents before or soon after the child is placed. You can ask the birth parents questions and they can ask you some, and hopefully you can get a photo together to show your child. The idea is that you and the birth parents can see each other as real people not just words on a page, that you can have some questions answered and some reassurance and also so the child later can see that you met each other and it was all fine. You can tell your child a bit about their birth parents from the advantage point of having met them, rather than only read about them.
The aim is to keep the lines of communication open. The birth family, if they are able to engage in contact (that's quite a big if tbh), can let you know how they are getting on, and let you know of any new siblings, any significant family events etc. On your end, you let the birth parents know how their child is getting on...letters are quite general, you might write abour how child is doing in school, their likes and dislikes, personality traits etc. Then the child when old enough can be included, so (theoretically) they can be reassured their birth parents are doing okay, that they can get any questions they have answered etc
How it works in practice totally depends on the situation. Some adoptive families do very successful contact which is very helpful for the child, in other cases contact doesn't have a lot of benefit, in some cases contact just breaks down completely. It depends on the attitude of the adoptive parents, birth parents and the childs desires and needs.
I did no contact with my oldest at her own request (older child), and letters, photos and meetings (between me and birth mum) for my younger two. The letters were very good - appropriate and helpful, beneficial for the children. It helped my middle child especially with - being reassured her mum was okay, being able to ask questions and get answers, and because she loves her mum a lot and needed to have a way of talking to her.
Now, DD and mum have regular meetings as she is 17 and they are 'in reunion' as it were. Facebook as well, the whole shebang. This is unhelpful for her (IMO), and causes problems, but it is what it is, you just cope with it. It isn't what happens when a child is first adopted, it just might happen in teenage years/young adulthood in this 'facebook age'. Contacting is easy online.