Can you think of any recent changes in her life since this started - household, medical, litter, food???

She may have turned finicky on you. Sometimes they decided they don't like covered litter boxes. You could try removing the lid if you have one of those designs. You are certainly scooping enough but you could also try adding a second litter box, one for poo and one for pee :).

Make sure you clean the place she soiled with a good animal poo enzyme cleaner, too.

No changes in anything I can think of. Nothing that would remotely indicate she wasn't feeling well. I'll try taking lid off litter box... doesn't seem to do much to keep her from scattering a little out every time she uses it. Will also try adding 2nd litter box to see if that makes a difference. If there is an up side to this "problem", it's that her litter box is on a fully tiled surface... nothing absorbent.

Yeah, I switched to big rubbermaid, clear, high-sided bins for my litter boxes. It prevents them from sweeping out paw-fulls of litter when they are covering up. It takes more litter to initially fill up but with scoopable litter nothing goes to waste. It doesn't help with tracking out of the box but it sure keeps things much more tidy.

Don't make any changes right now while you're trying to figure things out but you might consider them when everything is resolved. She's young enough where hopping in and out won't be an issue.

Cats are peculiar about their bathroom issues because in nature it puts them in a vulnerable state (from the scent).

Heard about using RM bins before. Guessing CLEAR so she will SEE what's there to begin with??

I use the box in the link below. My cat is only 1 yr old but she's tall and long and this is the only box she fits in well. All the other boxes, especially the covered ones don't allow her to squat correctly when doing her business.
It has really high sides but the lower front makes for easy access.

Here is a link that might be useful: litter box

Is she actually pooping outside the box? As in, are her feet in but she's not all the way? Or, is she trying to go in the box but she's getting poo stuck in her fur (or she ate a long hair- that has happened with our cats a few times).

Haven't actually seen this in progress... but sure it's not like her butt is sticking out the opening... neat little pile outside but close to box. Like I originally said, this is NOT an everyday occurence, but has happened maybe once a week over last month or so. June's "average" size, I guess. I took lid off box and added a second. Know it's only been a few days, but everything that's supposed to be in the box ended up iin the box.

Honestly, my opinion is that sometimes it's just something they do and there is no good explanation or reason for it.

Our cat did this starting at about the age of 4, and continued until he passed away just recently at 12 years old. Over the 8 years we did all kinds of medical tests, made all kinds of changes based on suggestions from other people, yet nothing ever changed the behavior. He did slowly progress from pooping more frequently just outside the box to pooping in other places in the house. As he got older he clearly had some issues with dementia, and this only magnified the problem. He had always had behavioral problems, and honestly sometimes I think this was also his way of acting out.

Best of luck to you - I hope you have better luck than I did!