It could be poor seating, or a design mismatch between the tire and rim. Most of today's HP tires need a hook edge rim to withstand the rated pressure, conversely, there are still a few designed around straight side rims, which can be difficult to seat on a hooked rim.
One way to tell the difference is to look at the tire. On tires designed to take full advantage of a hooked rim, the bead area bulges beyond the outer face of the tire slightly. That lets pressure push the bead under the hook forming a mechanical interlock. I'm not saying that a tire without a visible hook won't seat, just that it's not as secure a fit.
The other possibility remains poor seating, or trapping the tube between the rim and tire. Since you're experienced, I won't rehash tire mounting 101, but suggest that you consider mounting the 2nd side starting opposite, and working down both sides to the valve. The advantage is two fold. First it allows the tire to move to the deepest part of the rim's center well so you have more slack to finish. (tire must be massaged back to even distribution afterward). Second, ending at the valve means that if tube is trapped, it can be freed by pushing the valve in taking the tube deeper into the tire, then pulled down assuring that the tube is well inside the tire.
As you point out, different tires & different rims argues against a defect, so it's likely something you did, and you need to take your time, checking your work to prevent a 4th blow.
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