the seed feeder let you choose a certain amount of your seed income and the amount of xp gained is capped at 25% per second of the amount require to pass to the next sparow level.
Also the seed income could be the number of seed you gain at the sunflower platform (at once) or the seed per second gained with the sunflower platform or the total seed per second (with seed gained by fishing and the seed interest). But i thing it's the first option.
Base xp gain per 1 seed is 0.01, and increase with some sunflower upgrades, sparrow milestone and sparow rebirb xp multiplier. Your current xp gained for 1 seed was used to be displayed on the ui and may came back cuz the dev have planed to rework the sparow ui.
i don't rly know why there's this 25% cap but it's surely to prevent you to max the sparrow stats instantly.
When you hit this cap, it don't use more seed that it needs. Thats probably why your slider don't change the number of seed used (shown at the bottom left), and the number of xp gained (the number at the right). But they increased with every new level (with the cap).
the sparrow xp also increased with every seed picked-up by the sparrow and with the xp that gain the seagull (with the xp share). And those gain aren't affected by the 25% cap.
And finally, mitosis was changed in the last update and from what i've understand, it now only use your sparrow xp (instead of the drained one) and the drained xp is no longer reset by mitosis