But it has to. This is the code at that line:
float h = (float) t.getHeight();
where t is the texture that is directly feed into the method from the FrameBuffer's blit-method. However, some dispose or remove-methods clear that reference when executed in another thread, especially when finalizing the framebuffer. Might be that the old framebuffer get's finalized and that triggers the clear process...i'll see what to do to fix this...