Solved. The problem with the footer becoming un-anchored was apparently caused by our partner’s website, which was set to rotate to landscape mode. When they changed their setting, the problem disappeared.
We created an absolutely positioned div “off screen” anchored to the top of the viewport, that would provide a black background under the status bar area of X-series iPhones (for other reasons we had to make the status bar transparent). The redirect was un-anchoring that div. The solution was to move the anchor point so at least part of the div was on-screen (I used 100vh - 10px, anchored to the bottom of the screen. The div had a fixed height that extended above the top of the screen). Including here for reference in case anyone else has a similar issue.