Not *EXACTLY* your problem, but some time ago two of my Linux machines would get scanned first with a (incorrect) SNMP user/pass, and would not proceed to the SSH part of the scan.
The error was something like "device was not scanned, but previous SNMP info found" ... or something like that, I did not wrote it down, and it is not on the support page.
Running DeviceTester would get the correct information, so I messed around with the credentials order, and what solved the issue was placing the correct SNMP v3 credentials first on the list, and only them the SSH credentials. In 30+ machines, almost all equal in terms of config, only two had this issue...
So, in a nutshell, please check the credentials for the IP range, check the order, maybe do a new credential group (NOT a global one) with the correct info and place it on top of the list.
Actually, another issue, did you changed IP ranges ? An overlapping IP range could, in theory, do that...