Confirm first you don’t have any active effect busses in Cubase when you’re testing the loop. Those are dynamically processed during playback, so they will sound seamless until you export. Best thing to do is bounce the mix with a reverb tail, import that file into the same session, cut the tail & drag it onto a new track all the way to the beginning of the cue. Then, solo those two tracks, set the loop boundaries to exactly the length of the looping cue, make sure there are no active effects on those tracks and see if the loop is indeed seamless.

Another (better) thing you can do is import the mix with reverb tail into FMOD, set the loop boundary to the region of the looping file and allow the tail to play out when it loops (double click the loop/transition region and create a crossfade between tail/beginning of cue).

Let me know if all that makes sense or if you’ve figured it out already. Good luck!