Hi – it doesn’t look too bad; I would probably just spread a bit of Aquaseal over that area, or seal a scrap of tube fabric over it if you don’t mind the look of a bit of yellow there (it’ll be on the bottom of the packraft, so not visible when in use).
It could be the temperature is too high, but it looks good elsewhere, so maybe just avoid lingering so long in one place. It’s tempting to heat a large area to work those pesky bubbles towards the edge of the fabric, but it’s easy to melt through the floor if you do it too long, so if the bubbles are small just leave them there (the seam is way wider than necessary for adequate strength), or use a needle to make a small hole in the tube fabric to release the air from the bubble (but don’t push the needle through both fabric layers or you’ll end up with an air leak).