linux.threadexit2 failed on AArch64 Jenkins

Failed in a run in PR #3741

http://139.178.83.194:8080/job/DynamoRIO-AArch64-Precommit/412/console

 62/175 Test  #55: code_api|linux.threadexit2 ......................***Failed  Required regular expression not found.Regex=[^Sideline thread started
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Waiting for child to exit
Child has exited
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]  1.76 sec
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Sideline thread finished, exiting whole group
Waiting for child to exit

Looks like the "Child has exited" is missing.