Buprenorphine does not interfere with insulin. In fact it's the opposite. If he's in pain or has inflammation, that can raise the blood glucose numbers. Addressing the pain with bupe can lower the BG numbers.
Curves done at the vet's office can be misleading. My not diabetic cat once read over 220 at the vet's office one day due to stress, and 53 at home the next day. When the vet called about bloodwork, he was worried about possible diabetes.

If you can test at home, you don't need to do a fructosamine, which is just an average of values over a couple weeks. And you don't have to pay for curves either. You can buy a lot of test strips for that savings. I just once paid for a curve. Neko hated staying at the vet and growled at her for her tests. Vet's don't get a lot of training in diabetes, so how knowledgeable they are does vary. I liked my vet so stuck with her for other things, and managed the diabetes with help from here.
Here are some useful concepts to help you learn how Pablo works on Lantus, from the New to the Group Sticky Note:
Learn how YOUR kitty is responding to insulin:
- Onset - the length of time before insulin reaches the bloodstream & begins lowering blood glucose
- Peak/Nadir - the lowest point in the cycle
- Duration - the length of time insulin continues to lower blood glucose
- How to do a Curve - a simple explanation