fix long-standing race condition in `split` #1770

Job	Run time
CPU (Windows) (msvc, Debug)	8m 43s
macos-CPU (MacOS) (clang, Debug)	2m 27s
macos-CPU (MacOS) (clang, Release)	2m 13s
CPU (clang 13, Debug, ubuntu 20.04)	3m 43s
CPU (Windows) (msvc, Release)	8m 16s
CPU (clang 13, Release, ubuntu 20.04)	4m 13s
CPU (gcc 11, Debug, ubuntu 22.04)	3m 50s
CPU (gcc 11, Release, ubuntu 22.04)	3m 20s
CPU (gcc 11, Release, ubuntu 22.04, TSAN)	3m 48s
CPU (gcc 11, Release, ubuntu 22.04, ASAN)	4m 42s
CI (CPU) (MacOS)	1s
CI (CPU)	0s
CI (CPU) (Windows)	1s
	45m 17s

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