Custom Study Session should be always on top of all decks. It should be considered as a special deck. For example, if there is a deck named Constitution, then Custom Study deck is shown below it. But rather, it should always be in top in Anki applications.
I guess you are requesting an add-on to be made adressing your ordering preference.
Anki orders the decks by their unicode value in UTF-8. More or less is ordered alphabetically and case sensitive for English language. Capital letters come before lower case letters. The ASCII order which Unicoe also follows:
After that you’d need to know the unicode codepoints for the rest of the languages to see the actual order. Long story short, an strategy without add-ons is adding a character like
# at the beggining of the filtered deck names to force the sorting to be above the regular decks.