In the iOS Photos app, opening a photo and then using the "Edit" feature and the "Crop" tool to rotate the photo by 90 degrees appears to remove the zooming limit. After rotating the photo sideways, a second edit can be done to rotate it back to the correct angle, leaving the bug intact and allowing for further zooming.
Unlimited zooming is only available on a photo that's been rotated through the Crop tool, and exiting the Photos app or switching over to another image resets the photo so the zoom bug no longer works. A video demonstration is below.
Zooming in beyond the current thresholds in the Photos app is of little practical use in most cases, because the image becomes extremely pixellated, but it could be useful to people who want to zoom into a specific area of an photo just a bit further.
The zooming bug works on iPads and iPhones running iOS 9.3.2 and the iOS 9.3.3 beta. It is not clear when it will be fixed as it's likely a bug that's not a main priority.