You’ll generally be able to find out if it’s diet fatigue causing significant water retention and masking fat loss by simply implementing a refeed and taking daily kcals significantly over upper threshold of maintenance (at least 500-1000 kcals). And really. These refeeds won't do what they are intended to do unless the kcals are significantly over maintenance. Don't be afraid of the BW spike. Even on paper a 1000 kcal surplus results in negligible fat gain. I find the tighter the window you can do the refeed in the better. The day after the refeed you can capitalize on the additional reserves and transiently increased RMR by dropping kcals moderately lower than your base dieting days. If it is diet fatigue, once you jump back on your low days (assuming your low days are low enough and not just lollygagging at just slightly below maintenance), you should drop below baseline in like ~3 days.
Diet fatigue does contribute to water retention. Water retention masking fat loss isn’t necessarily a bad thing in and of itself. But diet fatigue ALSO increases risk of muscle loss. So water retention is a good proxy for risk of muscle loss.
The 500-750 kcal rec Mike talks about is actually a good rule of thumb for bodybuilders (you’re not a fat mess). Accounting for the fact that BW loss isn’t purely fat loss, it translates to about 2 lbs of BW loss per week which is as fast as you should shoot for unless you just don’t care about preserving muscle.