as an aside, i'm pretty puzzled that this worked in a dev but not a prod build! wondering what's to explain it

Production react doesn't do any error checking on the children. Development mode does. In development mode that checking causes a forced evaluation. The normal Dom stuff forces children as well, so it is difficult to run into this particular problem unless you're lazy sequence isn't embedded within a dom call.

i see--yeah, in my fulcro components where this was happening, I wasn't using any fulcro dom elements, so that would do it i suppose

You could also use mapv, which isn't lazy...

https://github.com/fulcrologic/fulcro/discussions/477 I thought github discussions were googlable… :-)