I've had that happen once when I was using it on Windows with Wezterm.
It depends on how you paste. I'm not 100% clear how it works, but I believe micro reimplements pasting directly with xclip (idk what it does on Windows) so if you paste with Ctrl+V, it just instantly inserts what's on the clipboard as a block and copies the source indentation. If you do Ctrl+Shift+V, sometimes it essentially just gets the clipboard piped from the terminal each character one at a time, not knowing what comes next, so then it indents extra when it sees the return character and then it naïvely gets a couple tabs added on top.
It depends on how you paste. I'm not 100% clear how it works, but I believe micro reimplements pasting directly with xclip (idk what it does on Windows) so if you paste with Ctrl+V, it just instantly inserts what's on the clipboard as a block and copies the source indentation. If you do Ctrl+Shift+V, sometimes it essentially just gets the clipboard piped from the terminal each character one at a time, not knowing what comes next, so then it indents extra when it sees the return character and then it naïvely gets a couple tabs added on top.