This is the first time I have ever read or heard that going deeper can increase fatigue.
I'd like to hear about this previously unheard of (in my case) phenomena.
It's not the going deep per se, it's just nitrogen loading however you get it. Going deep is the fastest way of course.
Try doing a dive where your nitrogen loading is close to the NDL's and then ascend at 30 ft/min, do a 3 min stop at 15fsw and get out and repeat all day if you are doing multiple dives.
If you feel tired and it's not just because you had long surface swims or long walks in heavy equipment to get to the water then try the same dives with 15 minutes spent (at the end of the dive) just messing around looking at small marine life in the range 30 fsw to the surface. You will most likely not feel nearly as tired after the dives.
In effect you are allowing your body to decompress even though there was no decompression obligation. Your body still has to offgass even though it didn't exceed the DCS limits.
It's just a matter of find a dive profile that works for you. I try to end all dives this way. Along with doing the deeper part of my dive first it just makes the rest of your dive one long decompression stop (s).