It looks like an example of fasciation.

Here is a link that might be useful: Fasciation

Flora, thank you!!! How bizarre and fascinating!
Hmm...once you read "mutation" it suddenly doesn't sound so appetizing, haha. I wonder if these leaves are okay to be harvested assuming they continue growing?

The only odd thing that all the shiso in this pot experienced was what looked to be a lack of calcium absorption early on. It affected them up until the start of the second set of leaves when I moved them out of that pot and into this one where they have now been for 5 weeks or so. All the growth since then softened up and became normal. I pruned the tops of all others in this pot (which then resulted in leaking stems for a week!?!) except this one, where I wasn't sure what it was doing or where/how to chop.

This same plant is growing one ridiculously large shoot/stem, enough to make me think a seed had inadvertently been planted there, except it is connected to this plant, I found, once I dug in a little.

Fascinating stuff! Thank you for identifying this!

Grace

This plant continues to mesmerize me. I harvested no less than 30 leaves off of it over the weekend, and underneath all of that are so many more small ones busting out. The photos below were taken today, after the leaves were harvested.

This fasciation business is so cool. Here's a shot of a plant from the same seed planting. Only one turned into this beast of a plant; the rest are as expected and normal-sized....and slowly growing.

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