What are your parameters (with numbers).
Your tank is still very new, so I wouldn't be too surprised to see some of these goofed up a little.
What are your lights? (type, spectrum, wattage, number)
Where did the nem come from? (store, local, wild/propagated)
No, this is not normal. Even if you got more powerheads BTA's will not just let go and float around. If it was a flow issue, they are far more likely to crawl across the rock. It is very dangerous (in the wild) for a BTA to completely let go - predetation, getting ripped on a rock, never finding another rock again etc - so this is a sign of something major going on. You say you've had it for a few weeks, so you put it in there when the tank was 2 months old? That's really pushing it, even for and old pro with tons of experience. Can you take a picture? Often times we might be able to see something about the anemone that may not register to you.