reboot needed to refresh MTP list?
While I do not have an answer for this strange behaviour (which I'm experiencing as well), I can say that a whole reboot may be skipped. In most cases, in fact, renaming the file you wish to transfer will make it visible on your PC as well. Additionally, if you wish to embark in the journey of installing adb on a Mac, you'll find that:

adb pull "android_file_path" "path_to_copy_the_file_to"


will be able to copy the desired file to your computer, regardless of it being visible on Android File Transfer or not.