Bags keep floating halfway through a twelve hour brisket cook Equipment

Nothing is wrong with your seal, that is water vapour and rendered fat expanding inside the bag as the meat heats and gives up moisture. Every long cook does it. Two fixes that actually hold:

• put a rack, a heavy ceramic plate or a couple of dinner knives in the bag's way so the bag physically cannot rise. A cheap wire cooling rack cut to fit the container is the standard trick.
• clip the top of the bag to the side of the container with a binder clip so the trapped air collects above the waterline rather than lifting the meat.

What you should not do is ignore it. Meat above the waterline is not at temperature and for a twelve hour cook that is a food safety issue, not just an evenness one.

Common cause of a weak seal on wet meat: moisture in the seal channel. Pat the top four inches of the bag bone dry, or freeze the brisket flat for 30 minutes before sealing so it does not weep while the machine runs. Half my early failures were this.

If the bag is genuinely inflating rather than floating on trapped air, that is worth a second look. A pinhole from a bone edge or a poorly sealed corner lets water in slowly, and the tell is that the bag feels heavy and the water looks cloudy. Double bagging anything with a bone or a sharp trimmed edge takes ten seconds.

Ping pong balls floating on the surface are the low effort answer to evaporation if you do not have a lid that fits. Looks daft, works well, and they store in a jar.