This topic is so difficult to discuss because everyone’s physiology is so different along with their varying training history. What is true for someone else may be the exact wrong idea for you/me.
I personally had a nearly identical experience as the OP. Some of it was related to the lack of racing last year, i.e. no specialty taper due to no events, but I can’t blame it all on that.
I made it through multiple iterations of the base/build cycle with good FTP improvements, increased endurance, new PRs, etc, before I hit a wall. My body felt like I had a mild viral infection, but I didn’t have any of the coughing/snottiness/etc, just this vague feeling of tiredness and malaise, muscle pain during workouts that was way out of proportion to the intensity and volume, sleep being “OK” but not great. After a rest week, I would feel better pretty quickly, but then it would come back after just a few workouts in the next plan I would try.
After about the third cycle of this, my wife recommended I take a full month completely off the bike, and it helped tremendously. She supports my training, but also saw that I was inside my head a lot (if that makes any sense). Looking back on it, I was doing too much volume for too long without a real “off-season.” Indoor training and TR makes high-level training year-round possible, and I had neglected to take a full break. I am now feeling much better and am back to steady consistency and building form.
I was feeling bad enough that I decided to really take the volume to zero, but I have wondered how I would have felt if I limited that month or two to just endurance rides with no intensity or intervals. I am now being much more careful about pushing into the “red” for consecutive weeks/months at a time. When people talk about this stuff, they frequently do so in the context of taking a “rest week” or going through a few weeks of intense training. For me, it took about 1.5 years to really get in the hole, which was a problem in itself, because I had a decent training history that had given me the wrong impression about how much volume I could handle. Every time I would try to start back after a few days off, I would look at my history of 10-12 structured hours a week for over a year and say “I can handle it now because I handled it then.” Big mistake.
Lots of people will say “of course” to this story, but for me, it was a radical shift to move from other endurance sports to cycling. The coaches repeatedly talk about taking time off, but it’s hard without the experience to monitor your own body. I previously did marathons, and I could do some pretty serious volume (50-70 miles per week), but I would have to slow down after a heavy week due to sore ankles, feet, back, neck, etc. With cycling, I think it is much easier to push yourself into these states because there aren’t a lot of limiters like there are for running, swimming, weight-lifting, etc. I tell my coworkers who do endurance sports that I regularly pop off 3000 kJ workouts, and they are appropriately flabbergasted, at which point I realize, Wow, cyclists are pretty nuts 
Cyclists, especially those of us without nutritionists, with jobs, and without professional contracts, need to be more careful about this IMHO.