Fix TS Observers type #4737

Job	Run time
ci (ubuntu-20.04, gcc, g++, true, 20, Release)	34m 57s
ci (ubuntu-20.04, gcc, g++, true, 20, Debug)	25m 28s
ci (ubuntu-20.04, clang, clang++, true, 20, Release)	28m 54s
ci (ubuntu-20.04, clang, clang++, true, 20, Debug)	22m 21s
ci (ubuntu-22.04, gcc, g++, 20, Release)	30m 52s
ci (ubuntu-22.04, gcc, g++, 20, Debug)	21m 23s
ci (ubuntu-22.04, clang, clang++, 20, Release)	30m 38s
ci (ubuntu-22.04, clang, clang++, 20, Debug)	24m 20s
ci (macos-12, gcc, g++, 20, Release)	6m 32s
ci (macos-12, gcc, g++, 20, Debug)	11m 2s
ci (macos-14, clang, clang++, 20, Release)	4m 24s
ci (macos-14, clang, clang++, 20, Debug)	4m 12s
ci (windows-2022, cl, cl, 20, Release)	7m 26s
ci (windows-2022, cl, cl, 20, Debug)	5m 43s
	4h 18m 12s

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