The problem you describe is a design decision and is working as expected. If you need to reimport the system into SMS, use the "Discover Intel AMT via IP Scan" function.
If you are using the SMS Add-on for AMT in "Integrated with SCS" mode and you perform a client discovery using SCS, the Add-on records the time/date when it performed the discovery.
If you perform another discovery using SCS at some future date, the Add-on will only discover systems that have been provisioned since the last time the SCS discovery was run. As indicated by the first reply to your post, this was a design decision to optimize the discovery process by not re-detecting existing systems and only detecting systems that were provisioned since discovery was last run
In the case you mentioned, since you deleted the client resource from SMS and re-ran the SCS discovery it will not be re-detected by the Add-on because the client was not provisioned since last time discovery was run
If you want to avoid using an IP scan you can reset the last discovery time / date - there are some entries in the registry that are fairly intuitive to change although as always, you should exercise caution editing the registry