So after that . the group product you have created is exist ? or its also disappeared ?
can you please do reindexing by running this command - php bin/magento indexer:reindex and then check ?
Also if you check the error logs and post here its help us to troubleshoot the issue !
Grouped product stays but the simple products added to that group disappears right after saving it.
No, reindexing did not fix it.
exception.log doesn't have anything entry related to it
looks like you are trying to add some other product type into grouped product and that is why its disappearing from there !!
So when you are adding products into grouped products - make sure you add only simple products in it - product type must required simple product only !
Also those product have quantity more than 1 !
Please try this and confirm working or not ?