I think i found a rendering bug or something like that in the AdvancementsScreen class. As far as I know, Forge adds a few things so that the AdvancementsScreen also works with more than just the vanilla advancements and adds buttons to navigate in the tab pages. But the super method during the render method isn't called so the buttons won't be rendered, it isn't game-breaking but i thought it wouldn't harm to report it.

Code from AdvancementsScreen#render :

public void render(PoseStack p_97361_, int p_97362_, int p_97363_, float p_97364_) {
   int i = (this.width - 252) / 2;
   int j = (this.height - 140) / 2;
   this.renderBackground(p_97361_);
   //SUPER METHOD MUST BE CALLED HERE FOR BUTTOSN TO RENDER
   if (maxPages != 0) {
      net.minecraft.network.chat.Component page = new net.minecraft.network.chat.TextComponent(String.format("%d / %d", tabPage + 1, maxPages + 1));
      int width = this.font.width(page);
      this.font.drawShadow(p_97361_, page.getVisualOrderText(), i + (252 / 2) - (width / 2), j - 44, -1);
   }
   this.renderInside(p_97361_, p_97362_, p_97363_, i, j);
   this.renderWindow(p_97361_, i, j);
   this.renderTooltips(p_97361_, p_97362_, p_97363_, i, j);
}

 

Screenshot 1: vanilla AdvancementsScreen

Screenshot 2: Custom AdvancementsScreen with the super method called durin render

No idea if this is an bug, make a pull request to the forge repo if you think this is an bug