First, on projects as large as Instagram, back engineering design decisions is tricky because there are always too many variables.
In this case, the circular button seems a solution imported from mobile, where click area is important. On desktop, not so much, so I'll say that focusing Instagram web towards desktop could be a reason.
Also with the circular button you lose a lot of space, so making the UI more compact and making better use of space could be another one.
Making the UI more consistent could be a third one. Usually primary buttons — like Follow — have a blue blackground and secondary buttons — like Following, to unfollow, or Edit profile — have no background and an outline. Both secondary button examples are used when showing an action different from the main action — or default — in the same place, to reduce the chance of clicking by mistake. As the Load more button is the only one displayed in that place, showing it as a primary button seems appropriate. Also the circular button shape wasn't used anywhere else on the UI.
To be consistent with their mobile app.
If you noticed, recently, they changed their button styling to have full-blue bg coloured buttons for inactive/not-pressed buttons and hollow (border) ones for active/pressed buttons. This was not the case in the past, they had hollow buttons (round button in your screenshot is an example).
I guess this change has been applied to their web interface now.