If you build something out of vanilla logs, (ex. Acacia, Oak, ect.) and mine one log with an axe; it veinmines the whole structure, only when you are not crouching. However, if you crouch while mining it, then it doesn't vein mine it. It only does it with vanilla logs, it doesn't do it with any of the modded logs. I checked everywhere in the keybinds to see if it was a keybind I missed, or maybe a mod, but I found nothing. It also seems like it only does it if the log is touching dirt, as if it were a tree. May be a feature but I would had hoped if it was there was a way to disable it, but I found nothing.

EDIT: it seems to only happen with any vanilla axe. Also, I believe the mod causing the issue is Tree Harvester.