It helps to walk rather than sprint or sparrow into the lost sector. If enemies don't spawn, make your way to the final chest, then backtrack the way you came in. If enemies start spawning in then you should be good to run through the lost sector like normal. Otherwise while backtracking and you get to the area where the first set of enemies are supposed to be, walk (don't sprint) towards the entrance and look at the left corner of your screen for a message that says "The seam between realities grows weaker..." before you get to the first set of rocks you'd need to jump up to get out. If you see "Rheasilvia" appear before seeing that message, turn around and try again. If on another attempt you don't see the message appear by the time you get through the first set of enemies (or they don't spawn), you know it hasn't worked.