4x6's will shuck the blade off the bottom (drive) wheel) when cutting even if it tracks ok unloaded, if the bottom guide is too far forward (bottom rear support roller pushing into the back of the blade). It did it on mine when the bottom rear guide was ~1/4" too far forward.? Probably be the same for your 7x12.

What I think happens is the bottom rear guide roller acts as a fulcrum; push the blade in above the roller with the pressure of a cut, and it will bend out below the roller, then?it?comes off the drive wheel.

When the blade is tracking properly the rear support rollers (top and bottom) should run?just?clear of the back of the blade, the pressure in the cut will bend the blade back to firmly seat it on the rollers, and the cone on the wheels will be driving the blade into the cut..? At proper tension on a 1/2" wide 10-14TPI blade, it bends up about 0.040".? Don't know what it is on a 3/4" blade, will be less at same guide separation, but more than 0.020" would be my guess.

Given that blades are hardly ever dead straight, just hitting and missing the rear rollers is ok.? Don't want them running on the rear rollers all the time.

Start with the rear guide rollers not touching the blade, then use the tilt to track the blade on the top (driven) wheel first, and then position the drive wheel on its axle to track the blade on the bottom, then bring the guides in so rear support rollers are?just?not touching - jv