If you look back at Sommers vs Gaban the first wound Sommers suffers is that Gaban cut off his hand.
Gaban then proceeds to then chop Sommers into more pieces before running to Gunko. However when Sommers does get up the first wound he receives is the last one he healed from. Based on this chapter it seems to be because the rest of Sommers body reattached itself and Sommers hand didn’t regenerate until later when his regeneration reactivated after being hit with Gabans CoC.
There was no retcon, this is how it’s always been. All HK have regeneration, however, when they take attacks that counter regeneration they can still reattach their body parts. But until their regeneration reactivates, they can’t regrow any missing parts.