Not a bug! Say I have two clips that are 4s each. If I place them next to each other in the timeline (for a total of 8s) and I want to add a 2s cross-fade between, I would need 1s of extra footage from each clip. Since the clips are only 4s, there is no more information with which to cross-fade. In the attached image, the clips on the left represent non-trimmed footage and the clips on the right represent trimmed footage with a crossfade. The bottom track is the clips as edited; the upper two tracks represent the full-duration clips. In the left example, clip 1 ends as clip 2 starts, so clip 2 cannot start fading in before clip 1 stops and clip 1 cannot fade out after clip 2 begins; the frames just aren't there. In the right example, the upper two tracks show how trimming the clips in the bottom track left extra frames with which to cross-fade. These are commonly referred to as "handles."
Premiere Pro added a feature a few years ago that allows you to auto-crossfade your clips as you desire. Have an empty sequence open, then select your 50 clips in the Project panel. At the bottom, just to the left of the magnifying glass button is another button called "Automate to Sequence". Click it (this can also be found in the "Hamburger" menu in the Project pane), and you'll get a dialog with lots of options, including how much overlap (cross-fade) you want and whether to apply default audio/video transitions.
Hope this helps!
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