Decimate modifier is reducing faces, even though the set angle is smaller than the Angle Limit set The decimate planar reduces the number of faces depending on the angle.
But there's a 110-degree, but at 52 degrees, the face is reduced.
Why is the face decreasing when it's less than 110 degrees?
additional question.

180-129 = 51. So I see it flattened at 52 degrees.
But I have one more question.
Why is it decimated like the screen shot above?

Shouldn't the 52nd degree be straightened out?
Shouldn't it be straightened out like the red part?

It's less than 35 degrees, and it's getting decimate.
A test case:
From left to right: 90° then 30°.

If we go above 30° in the modifier the 30° angle is decimated:

Now we have a 105° angle:

which means the angle variation is 75° (180 - 105°) following the edge (or face) variation.
So if the modifier angle goes above 75°:

... the edge collapse again.
That means the modifier is &quot;cumulative&quot;: it will decimate all parts where the angle variation is below the given angle in the modifier.