That's the oddity. My colleagues were also logged into the document and the icon was displaying their initials so I knew it was them but the unknown guest accounts that were not showing up in 'Manage Access" had names such as "Melon", "Papya", "Sycamore", etc. names of fruit and things. Another user within this issue also reported a "sycamore" name as one of the guests which is interesting. Given that case, I'd expect it to be something internally with Microsoft.
Spoke with Microsoft tech today and mentioned they may have an unknown security issue. The tech was unable to speak to the issue but will address with IT Security team. Maybe we'll hear something more. Maybe not.
Nonetheless, I wasn't going to wait for their response to say "it's all safe", I moved everything off OneDrive.