The way Lansweeper automatically determines asset types is quite complex, but to quickly summarize this functionality: Lansweeper connects to an asset and checks which protocols are available, checks the contents of headers, the results of commands, etc. and uses this information to determine the asset type.
Likely what's causing this discrepancy here is the difference in protocols available on either asset. It's not entirely clear to me if in the Linux UTMs case actual information is being retrieved through SSH or not, but if that's the case and the other asset is identical, enabling SSH on the device may restore parity.
For troubleshooting connectivity issues for non-Windows assets (which includes the determining of asset types) you can make use of devicetester.exe which can be found in the Program files (x86)\Lansweeper\Actions folder on your Lansweeper server. Make sure to run the test directly from your Lansweeper server for an accurate simulation.
On a side note, since you indicate the local UTM is in the local subnet (presumably the same subnet as your Lansweeper server), it may be possible that no protocol is available at all apart from ICMP and that it's still getting scanned. For devices in your Lansweeper server's subnet the MAC address can be retrieved from the ARP table of your Lansweeper server, from which some additional information can be retrieved in return. This is enough data for an asset page to be generated.