Why do I get tons of RUNNABLE threads without stack?

Issue

I got thread dump when my application slowed down significantly under load test. The thread dump shows more than 50 RUNNABLE threads without stack.

"WorkManager(2)-213" daemon prio=10 tid=0x000000005ce28800 nid=0x48d4 runnable [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE

"WorkManager(2)-212" daemon prio=10 tid=0x000000005a8db000 nid=0x489e runnable [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE

"WorkManager(2)-211" daemon prio=10 tid=0x000000005cbc2800 nid=0x489d runnable [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE

The application didn't hang, but was very slow.