I am starting to think it's either a bug in logic....
I think it is worth pointing out that.........
Over the years many users have reported such an issue and in every case i am aware of, where the user resolved the issue and reported back... the problem itself was found to be caused by;
1) Incorrect settings in the playback app... (The volume control for that app was set low for example)
2) Sound enhancements were being used in the playback app (Example: iTunes's Sound Check / Sound Enhancer functionality being turned on in it's preferences)
3) Playback of audio via different hardware settings (using an external audio device and the mixer for that device was incorrectly set)
4) The use of the Normalize function during bouncing in Logic .. (Uncheck it)
5) The use of Audio Routing apps on the Mac.. such as Jack or SoundFlower for example, where again, the volume setting in that app was different/lower resutling in a lower volume during playback outside of Logic.
6) Use of a 'faulty' 3rd party plugin that doesn't not function correctly during offline bouncing (Bounce in realtime to confirm)
7) User Error of some other manner... Everything from, listening to playback via different speakers/headphones/different location to simply believing it sounded different because ears and hearing changes over time!
To test....
a) Create a bounce in Logic to a standard wave file
b) Create new project
c) Drag and drop the bounced wave file into Logic's arrange window
d) Playback onside Logic and check volume levels... on exactly the same hardware/location used with Logic.
Note: This is not to say there isn't the possibility of there being a very rare bug in a version of Logic that could cause this issue but if there is such a bug, I have never come across it myself, nor have I ever come across anyone else confirming such a bug at any time in the past. It has always been proven to be one of the above mentioned problems that has been found, in the end, to be the cause.