Unable to ptrace(PTRACE_TRACEME) without sudo

The call to ptrace(PTRACE_TRACEME) fails in gdbserver in spite of ptrace_scope being 0. As a result gdbserver hangs and is impossible to use. When run with sudo it works fine. This seems to be a kernel bug.

# which gdbserver
/usr/bin/gdbserver

# cat /proc/sys/kernel/yama/ptrace_scope
0

# gdbserver :9091 /bin/true
<no output>

# sudo gdbserver :9091 /bin/true
Process /bin/true created; pid = 31071
Listening on port 9091

# strace -f gdbserver :9091 /bin/true
...
ptrace(PTRACE_TRACEME) = -1 EPERM (Operation not permitted)
...

# getcap /usr/bin/gdbserver
<no output>

Adding "setcap cap_sys_ptrace=+eip" to gdbserver doesn't change anything.

Further details:

# lsb_release -rd
Description: Ubuntu 18.04.5 LTS
Release: 18.04

# apt-cache policy linux-generic
linux-generic:
  Installed: 4.15.0.118.105
  Candidate: 4.15.0.118.105
  Version table:
 *** 4.15.0.118.105 500
        500 http://aptrepo/ubuntu bionic-updates/main amd64 Packages
        500 http://aptrepo/ubuntu bionic-security/main amd64 Packages
        100 /var/lib/dpkg/status
     4.15.0.20.23 500
        500 http://aptrepo/ubuntu bionic/main amd64 Package