Maybe a reaction from the repotting. Makes sense.maybe from root stress?
Thanks for the info, this will help people figure out what's going on. Other people, because I'm out of my depth.Tree was collected from yard 18 months ago (spring of 2022) and placed in a starter pot. Over-wintered and came out strong this year. Up-potted in May, continued to thrive. Snipped off an unwanted leader (not the top apical leader; see very rough drawing) roughly three weeks later. Growth slowed somewhat but not unexpected. Substrate is free-draining Bonsai Jack's soil (zero puddling or overflow; straight through). No smell of root rot; no fungus gnats. Thank you peeps!
I'm not well versed on how fungal diseases work other than those that are transferred directly to the leaf surface (topical), which is pretty common. Basically, this looks systemic rather than topical. With that in mind, I suspect it became infected when I made that cut. It was the lowest branch on the tree, so it's difficult to pinpoint since all branches above it are infected... but decreasing with the higher branches. I'm usually pretty diligent in keeping my tools clean but have become somewhat lax this year. I honestly think that's what happened... that I introduced a fungus or bacterium when I made that cut. I sealed it, but that didn't matter if the infection was already introduced. Anyway, I appreciate the peeps who responded and I hope this weird little thread helps others. Thanks all.Appears to be one of the various fungal leaf spot diseases common to maples