As explained by Brad16840 on the Minecraft.net forums (https://www.minecraftforum.net/forums/mapping-and-modding-java-edition/minecraft-mods/1291844-backpacks-by-brad16840?comment=1062), the event for item pickup fires much later than backpacks as it was set with a LOWEST priority compared to backpacks NORMAL priority. By that point, the backpack has already cleared the item queue and the HUD doesn't see it.

Is there a reason it was set to LOWEST instead of HIGHER or HIGHEST? I would personally like to see items going into my backpack or other item storages when I collect them.