Yes, two of my last four brews ( all ESB's). I haven't tasted the other two yet. In my case, there are a number of possible reasons.
1. I switched from Munton's crystal to some other brand. That may or may not have an effect.
2. I've reduced the amount of crystal and cara-pils by 25% each.
3. I've been mashing a bit thinner, and the trwo that I have tasted were mashed a bit cooler, because I hadn't dialled in the correct strike temperature
4. I thought I'd run out of my favorite yeast (WLP002), but I found a tube hiding in the fridge which will be put to good use tomorrow.
Tomorrow, I'll go back to the thicker mash with more cara-pils and crystal, so the only difference will be the brand of crystal, and in about 3 months, I'll brobably have a good idea what the real cause was.