What happens if an eviction notice is put on a door, but is immediately removed and tenants never see it?

Depends upon other circumstances. In many states there is a "nail and mail" statute, meaning they attach it to the property (door) and ALSO mail a copy to the tenant. Proof of having done both means "service" is proper and the case may continue, regardless of whether the tenant had actual notice. The theory is that SOMEONE would have seen it and told the tenant about it.

In any case, the tenant will find out when they have ignored the notice they claim they "never got" and they get served with a summons and complaint for eviction by the court, or they default on that as well and get served the order of eviction, in the hand of a law enforcement officer.

They still get evicted.

your still moveing out , so pack up . get ready to move

Legally speaking the tenants would have a case to suspend the eviction, if they can prove they never received notice. Worst case scenario is the eviction court would be rescheduled and the tenant would fall into further debt with the landlord.

It remains in force.

Posting the notice to the door is legal. The landlord is not required to get any proof the tenant actually received it.

If the tenant removed it, it still exists. If it is a court order it will be enforced.