You may have noticed that on this site, I use 'Changgeng', rather than the more commonly used 'Chang Geng'. The main reason for this is fairly straightfoward - this is the correct romanization of his name under hanyu pinyin rules (the romanization system used in mainland China, and for all other names on this site as well). I think it also highlights the incongruity of his name in comparison to everyone else's, too - he doesn't have a family name or a given name until he's thirteen, and over the course of the novel those closest to him continue to refer to him by his childhood name alone.
How did 'Chang Geng' become the standard in fandom, then? Part of it has to do with the first fan translation of Sha Po Lang into English - rather than using Hanyu pinyin rules for name romanization, it instead used the romanization pattern more common in Southeast Asian Chinese communities, where the romanization for each character in a name is split up, rather than joining the sections based on what part of the name they're in (such that each part of the name is one 'word' in English). Under this system, 'Chang Geng' is in fact the correct romanization, and the translation is consistent in its use of systems.
The fandom, however, largely converted names back to Hanyu pinyin equivalent (such that 'Shen Ji Ping' and 'Gu Zi Xi' became 'Shen Jiping' and 'Gu Zixi'), but... didn't for Changgeng. It's hard to say why, though my theory is that non-Chinese webnovel fandom in general got so used to always referring to a character with family name and given name, that a clear single name felt 'wrong'. And then got so used to it that any change also felt 'wrong'.