Based on some reader feedback, it appears I have to recant. It appears the סגול might actually be correct.
Yaakov Gross wrote:
The reading with segol appears to be correct. Consider a regular Kal verb, say G M R (to complete), in the Nif’al:Nigmar with a patach is Perfect (aka “past”): it became depleted, it was completed;with a kamatz, it’s the present participle: it is completed. For nachei lamed he, the Perfect has segol in place of patach. Thus, laaseh nes – “a miracle was wrought”.Whereas the present participle keeps the kamatz, so na’asah would be “a miracle is being wrought”Some interesting sources from another reader:
See נחמיה ה:י"ח and ישעיה כ"ו:י"ח and contrast with ויקרא ז:ט.
See also אבן עזרא בראשית א:כ"ו and י"ב:ז.However, in a separate discussion, I was shown this extensive write-up on this very issue which seems to suggest the other way around and that the קמץ is correct.