[Notice: this is not an actual answer because I'm currently having the same issue, I just wanted to confirm you're not the only one and add some extra info, hoping to find a solution. Unfortunately I cannot add a simple comment as I don't have enough rep, feel free to remove my "answer" if you feel like it's inappropriate]

I'm having the exact same issue, the code used to work perfectly until a few days ago, now the InfoWindows do not appear in the iOS version of the app while they still work fine in the Android one, is that also the case for you? I've ran all your checks plus:

1. Added an onTap property to the markers themselves with a function printing some debug text, it works as intended (in addition to the "camera" centering on the marker)

2. Updated the google_maps_flutter package to the latest version

This is incredibly inconvenient as no errors whatsoever are displayed and it appears to have started behaving this way without any change in the code or package update.