I just got my Fleks3d plate today and my first print (a difficult model that failed to adhere to my glass & glue-stick build plate 6 times) finally printed and stuck.... and like you, it stuck TOO WELL. It fused with the plate. The instructions said no heat, or if you use heat, no more than 37 C. So I printed at 36 C thinking this was going to be a challenging print.
Using a blade (sharp scraper) I finally got most of it off, but still had to sand it with emery cloth to get the rest off. But, my blade put some good scratches in the surface.
I wrote Fleks3D and explained what happened, with photos, and the owner/creator, Viktor, replied immediately. He agreed that the initial bond can be very strong on a new plate. He said not to worry, that I could sand the plate with 100 grit (coarse) sand paper and it would restore the adhesion properties. He also suggested I print without any heat. So, I sanded the whole plate with 80 grit sand paper (didn't have 100), and printed something a little less challenging (a 2'x3' bracket) and it printed flawlessly and popped off the plate with a tiny flex of the plate. Now, I'm printing that darn tough one again.... so far it looks good. We'll see how easily it comes off the plate this time.
So, no heat and sanding with 100 grit seems to have solved my issues.
I also manual level (because I have a UM2+). I think Flex3D recommends this, as auto-leveling systems can burn the plate.
Good luck! I hope you get it working more reliably. It is really satisfying giving a little twist/bend and having everything pop off cleanly.
MJ
Update1: Just checked the job.... 1 small piece popped off mid-print! Dang. Managed to pause the job and sticky-tac it back in place. There are 72 small pieces, so losing 1 isn't a big deal, as long as the failed part isn't left to make a mess of the rest of the job. Now stuck back in place, the job is proceeding smoothly. But, that tells me, either sanding it, or having NO heat at all, has made it much much less adhesive. I'm going to have to experiment with different temperatures with this sanded surface to see if a little heat would help the sanded surface regain a little of that extreme adhesion the new plate had.