The activity you are describing is commonly called "blaming". Magit's manual contains a section about blaming.
The easiest way to show such information is to type C-c M-g b in a file-visiting buffer, but there are alternative approaches as described in the manual.
(Disclaimer: I don't know if this is how Magit and Spacemacs actually work, but it fits my hypothesis.)
When you magit-checkout a remote branch, magit can't know if the current file you have open in the buffer also exists in the remote branch. However, git-checkout changed the file, so a workaround (pointed out in a comment by MTS) is to revert the file (...