object.matrix_world doesn't update correctly

Question

I'm using a function to render thumbnails of some grease pencils, and in order to frame it well, i use the object.matrix_world to get the world coordinates of my points.

def render_scene_frame(frame_number, gplib_data) : #gplib_data is a property group (stored in another scene)
    print('render frame ', frame_number)
    mode = gplib_data.mode
    scene = bpy.context.scene
    scene.frame_current = frame_number
    print('scene/fr, mode : ', scene.name, scene.frame_current, mode)
for ob in scene.collection.all_objects : 
    if ob.type == 'GPENCIL' :
        ob_matrix_world = Matrix(ob.matrix_world)
        gp_ob = ob.data
        print('****', ob.name, ob.data.name)
        print(ob_matrix_world)

#... some more code

i'm calling this function from an operator, here's the part of code :

if self.gplib_data.export_anim == True : #assume it's true
    self.gplib_data.export_mode = 'ANIM'
    render_scene_frame(1, self.gplib_data)
if self.gplib_data.export_poses == True : #assume it's true too
    self.gplib_data.export_mode = 'POSE'
    for frame in range(1, ((self.gplib_data.export_range_stop - self.gplib_data.export_range_start) + 2)) : #assume it's something like range(1, 5)
        render_scene_frame(frame, self.gplib_data)

According to me, it will "render" (or at least print objects matrix_world) frame 1 twice, then frame 2, 3 ,4. And it tries to. But the matrix_world print shows different values for both frame 1, which doesn't makes sense to me. Looking deeper i feel that the ob_matrix_world, is late by one iteration.

render frame  1
scene/fr/mode:  Template Scene 1 ANIM
**** _head _head
<Matrix 4x4 ( 0.9998, 0.0000, -0.0198, -0.7120)
            (-0.0000, 1.0000,  0.0000, -0.0000)
            ( 0.0198, 0.0000,  0.9998,  0.2139)
            ( 0.0000, 0.0000,  0.0000,  1.0000)>
**** _hat _hat
<Matrix 4x4 ( 0.9998, 0.0000, -0.0198, -0.7422)
            (-0.0000, 1.0000,  0.0000, -0.1300)
            ( 0.0198, 0.0000,  0.9998,  0.9114)
            ( 0.0000, 0.0000,  0.0000,  1.0000)>
render frame  1
scene/fr/mode:  Template Scene 1 POSE
**** _head _head
<Matrix 4x4 ( 0.9959, 0.0000, -0.0904, -0.6969)
            (-0.0000, 1.0000, -0.0000, -0.0420)
            ( 0.0904, 0.0000,  0.9959,  0.3414)
            ( 0.0000, 0.0000,  0.0000,  1.0000)>
**** _hat _hat
<Matrix 4x4 ( 0.9959, 0.0000, -0.0904, -0.7762)
            (-0.0000, 1.0000, -0.0000, -0.1720)
            ( 0.0904, 0.0000,  0.9959,  1.0350)
            ( 0.0000, 0.0000,  0.0000,  1.0000)>

I'm definetly missing something. Thanks for helping me !