Anybody issuing a problem when you put the link you want to share in the group and it disappear?
It’s probably removed by FB since they consider it spam. You can check it, just to to “support inbox” for your FB account, there should be a message, something like “we removed your post…”.
Why do you consider links as SPAM? Can it be by user reports?
Long ago I do not get links as spam in the help mailbox, but I’m getting blocks from “Posting and Join Groups”