Is this an intended quirk of Recyclers?

[pseudo pecan]

I noticed recently, that when it comes to recyclers, they are fully capable of outputting stacked items. However I also noticed that if the belt they are outputting onto, is full, even if its of the same item as being output, it will not try to stack items. It makes more sense to me, that it would try to stack items, that already exist where it is attempting to output, i.e. the image, because copper already exists there, it should add ONTO that copper, to complete the stack. Would this not make recyclers work more smoothly?

I think this would only apply to recyclers, anything else and it would not work well.

[stark harness]

They can only output stacked items when recycling scrap btw

as far as I know nothing else can add to existing stacks either so It's likely intended behaviour

[pseudo pecan]

they can output stack when recycling non scrap

[steep flower]

Recycling products go into the output slots. (Those aren't actually trash slots, as the item position depends on the scrap recipe.)
From the output slots, items are pushed out left to right. If the currently ejected slot has more than one item in it, it'll try to stack it. This is guaranteed to happen with recipes that yield more than one item of a type (e.g. ). Can also happen when the belt is partially full and causes some backpressure, but too much will clog the output and stop the Recycler.
As 8992 mentioned, item stacks on belts are "read only". Nothing can add to existing ones.

[dusty kettle]

Item stacking on belts does not support adding items to exists stacks on the belt. Only new stacks can be inserted.

[sinful wharf]

If you want the outputs to all be stacked then you should put it in a chest with a stack inserter and have the stack inserter wired to the chest, and also be wired to a in the other color that has -15 to item that could be in the chest.

That way the inserter will only pick up items that you have 16+ of and it will never get stuck

[steep flower]

You could lower the hand stack size of the inserter to decrease hoarding of less common items

[sinful wharf]

Steel chest has enough space that it isn't necessary

[steep flower]

or add another inserter transfering items orthogonally from chest to chest

necessary, no; it's a one time cost

unless quality recycling scrap

[dusty kettle]

Steel chests have so many slots now... Just use a higher quality one if you are concenred of it overfilling.