I've actually never run the game on a narrower aspect ratio than either 16:10 or 16:9, so I'm not sure how the game behaves on aspect ratios like 4:3 or 5:4, but your image certainly does look like it has some of the sides omitted.
Some more information could help narrow this down.
What is the FOV set to? In my experience this setting makes things more or less "fish eye" the higher you go, and doesn't actually add or subtract content from the sides, but I wonder if it's a possible factor worth asking about.
Is this a modified installation/profile in any way? I see non-vanilla stuff there but you appear to be on a server, so I'm not sure how much of that is coming from that.
Does this happen in all worlds/servers/profiles? Maybe try setting up a new profile (in its own directory) that isn't modified and see what happens.
What is the configuration of your display (resolution), and that of the game (full screen, borderless full screen, or windowed)?
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