Vladlen, I've checked your example and also got this behavior
on the current master under Gdb (Ubuntu 22.04 LTS).
On the query above server waits in this state (as shown by gdb):
Program received signal SIGUSR1, User defined signal 1. 0x0000562beb21c6ed in ExecInterpExpr (state=0x562bec781858, econtext=<optimized out>, isnull=<optimized out>) at execExprInterp.c:625 625 return state->resvalue;
Although, the process continues to work normally after gdb is detached from it.