Before subutex, there were other, lower dose buprenorphin pills (Temgesic to name one) so it's reasonable to assume the patent for buprenorphine-only medication was approved before subutex came out.
Also I'm pretty sure there was actually more than 2 years between tex & xone, but I'd have to check on this.
Also FDA is an american institute, is it not? And subutex, suboxone & others are marketed worldwide? There's a huge market for buprenorphine in europe. And the company that held the patent for subutex is european. So I don't see why they'd specifically want to "trick the FDA"... or maybe they would, but that definitely is not the reason Suboxone was made, else they'd have marketed it in the US only...
Buprenorphine hydrochloride was first marketed in the 1980s by Reckitt & Colman (now Reckitt Benckiser) as an analgesic, available generally as Temgesic 0.2 mg sublingual tablets