When I worked for a shade sail company years ago, I spoke to an engineer regarding the relative strength of round posts compared to square/rectangular posts. He said that square is weaker if the strain is not perpendicular to the flat sides i.e, diagonally across the square section. If diagonal, the post will start to crush and when that happens it becomes closer to a flat strip and is very weak.
There should be stuff on the Internet that explains such things. It all depends on where the stretch, twist, compression, bend is applied. It'll tell you if/when it is not strong enough. Let's hope it is not too much to suffer at the time. One test might be to jump up and down on it.