Maybe someone who knows more about the ISS than I do can answer this:

Naively, I would assume that there are airlocks between the different sections of the ISS. I would also assume that they would close these airlocks while doing the kind of work they are doing to repair the leaks.

So, assuming I'm right (and my assumptions might be wrong,) why do the astronauts need to shelter?

There aren’t even doors between sections. Airlocks are serious things, there is one or two for station for EVA. There are multiple hatches for docking spacecraft.

One of the innovations of ISS is larger docking adapter with bulkhead that is removed after docking. Russian section still uses hatches. All of the cables go through the docking adapter or hatch which makes impossible to close door or quickly disconnect.

If things go wrong, they're already in the vehicle supposed to bring them back. It might be upsetting to be 3 locked doors away from your best way to come back home

Well, I won't claim to know the answer, but "please do not move between different airlocked sections while this work is underway" sounds a lot like the definition of "shelter" to me

There are normally-open air-tight hatches between modules. Various utility connections and air ducts are normally run through the open hatches so it would take a bit of work to disconnect these connections before they could be closed.

Not exactly something you want to be doing under time pressure.

I think the service module is both structurally and functionally critical. If it is failing and you do not know why, catastrophic failure is presumably possible, not just some air loss. A hole or crack in the module is now apparently double the size it was until recently, that is a trend that presumably could continue to rapid unscheduled disassembly.

> Naively, I would assume that there are airlocks between the different sections of the ISS.

There are not. The airlocks on the ISS are either docking modules for spacecraft, for spacewalks, or for deploying satellites.

The crew shelters in the vehicles so that in case of an emergency they can evacuate immediately.

Compression loss can lead to a decompression of sorts if I had to guess... it is a vaccum out there. The force from a decompression can yield a chain reaction or strongly disrupt the entire station.

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