I have a weird behaviour on my mackbook pro, and I think it's related to KBM.
When I select multiple files and drag them, only the top one moves (whether it's moving files on the desktop or between folders).
It's been like that for a while, and I think it's KBM related as I tested it on a friend's computer and it started doing the same...
I can't imagine why KM would cause that, but you can easily check.
Stop the KM Engine and Quit the KM Editor App.
Then try your drag test.
I doubt it is related to Keyboard Maestro.
In Keyboard Maestro, select Help ➤ Assistance, and then select Something unexpected is happening and follow the instructions.
If this thing is not about keyboard, it's hard to be critical of KM.
Are there software like BetterTouchTool、Jitouch or something like that installed on your computer？