We have a Fibrament stone that we purchased after doing a lot of research on baking stones. However, I don’t have much to report on it because I used it maybe 3-4 times and now it sits in storage. I can relate exactly to the temperature problem you had. I had some of the least successful bakes in my sourdough history!
I learned that when I set my (gas convection) oven to say 475, when the red light clicks off indicating that 475 has been reached, that is NOT the temperature in parts of the oven further from the sensor and especially it is not the temperature of the Fibrament. When I test it with an infrared thermometer, it is nowhere near 475. As it is, it takes about 45 minutes for my oven to “reach” 475, but if I want the Fibrament to reach 475, I’d have to increase the preheat time considerably. I don’t have the exact data on all this, but I know it was enough to make me stop using it because it required excessive preheating.
Furthermore, when I left it in the oven between bread bakes for convenience as they suggest (it’s heavy!), I found that the oven took longer to preheat for everyday use because the Fibrament absorbed a lot of the heat.
If I ever want to try another stone, I will get a regular pizza stone and for sure one that is thinner than the Fibrament because it will heat faster, even if that means sacrificing something (supposedly) in the result.
If you are ok with preheating until you see that the stone’s temperature is what you want, you should definitely try again. For me, it wasn’t worth it - I can get pretty good results in a clay baker.
I’m really sorry to not bring you more positive information.