Why do my friends ignore me in group chats but invite me out all the time?

I have a few friends from work. We started out great. Everyone got along and we decided to catch up outside of work and get drinks, go for dinners etc.
For the last two months they have been completely ignoring anything I say in group chats on messenger. They chat amongst themselves.. write nice supportive messages to each other but completely ignore what I have to say. Then invite me out with them still all the time and expect me to show up even though I feel completely ignored.
They even come up and say hi to me at work and pretend we’re besties…. its really strange.

I really want to bring it up and talk to them about it but don’t know how to go about it.
Don’t know whether they even realise they’re doing it or are doing it in a nasty way for some reason?
They have invited me to so many things that I don’t even feel comfortable going to now because they don’t talk to me outside of work.
Why do my friends ignore me in group chats but invite me out all the time?
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