I do not understand it well, but I think it is probably the case.
I tried the following code with reference to your comment.
$MyControl.visible = true
$MyControl/MyAnimationPlayer.seek(0, true) # no error
By executing seek () after play (), no error occurred.
Thank you for testing the code and thinking together to solve the problem.
Your advice solved the problem.
Thank you so much, Zylann! XD
I also tried the following code, but it was not good.
$MyControl/MyAnimationPlayer.seek(0) # bad.
$MyControl/MyAnimationPlayer.seek(0, false) # bad.
After the second time, the last frame is displayed so it is not good.