I have pinned down the regression to a specific commit, thanks to @aDotInTheVoid recommending me cargo-bisect-rustc.

Output of cargo bisect-rustc

searched nightlies: from nightly-2024-07-28 to nightly-2024-07-30
regressed nightly: nightly-2024-07-30
searched commit range: 2cbbe8b...612a33f
regressed commit: 56c698c

cargo bisect-rustc -c rust-docs-json --start=2024-07-28 --end=2024-07-30 --script ./regr.sh

#!/bin/sh
set -ex

if [ ! -f ~/.rustup/toolchains/${RUSTUP_TOOLCHAIN}/share/doc/rust/json/core.json ]; then
    echo "File not found!"
    exit 1
fi