How to fix zoom in/crop when sequence and clip size settings are the same?

I've got 3840 X 2160 clips for a documentary project in a 3840 X 2160 sequence, and they're weirdly zoomed in. Set and scale to frame size do nothing, and adjusting the zoom level (i.e. fit, 100%, 50%, etc) only makes the clip smaller in the panel but doesn't affect the zoom. Adjusting the scale in effect controls does the same thing.

This is only happening for some of my clips, not all—these are ones I didn't shoot but got from another videographer, but other ones that he gave me shot on the same camera during the same time period are fine. I'm working with proxies, but toggling the proxies on and off does nothing. When I look at the clips in the source panel, they appear at their full regular size, but the second I drag them into any sequence—unless I choose create sequence from clip—it does that weird zoom thing. The sequence settings in create sequence from clip, where the clip looks normal, appear to be identical to my other sequence settings.

Anyone know what the heck is going on or how to fix this problem? Or is there something in the sequence settings I'm not looking at that could be affecting this? I'd really prefer to not have to re-import the footage or just make new sequences, since I'm already deep into this project and had the (very long) sequences marked up with markers, etc. (I know I'm an idiot for doing that before figuring out this issue, but I'm on a super tight deadline and made the mistake of thinking I'd just "sort it out later.")