The volume buttons do not work properly

[acc-bert]

The volume buttons do not work properly on a Raspberry Pi 3 Model B with a HiFiberty DAC plus.

When the buttons located under the volume dial are used the volume bar springs only UP even when the mute button is pressed the volume dial springs only up with steps of 38, 64, 86.

The issue also occurs with use of the REST API commands as described on "https://volumio.github.io/docs/API/REST_API.html"

I will pre-empt a video of the problem or the problem is already clear?

System info:
PRETTY_NAME="Raspbian GNU/Linux 8 (jessie)"
NAME="Raspbian GNU/Linux"
VERSION_ID="8"
VERSION="8 (jessie)"
ID=raspbian
ID_LIKE=debian
HOME_URL="http://www.raspbian.org/"
SUPPORT_URL="http://www.raspbian.org/RaspbianForums"
BUG_REPORT_URL="http://www.raspbian.org/RaspbianBugs"
VOLUMIO_BUILD_VERSION="517644ba70b07904a7cfd78158fa2a8b12e9b934"
VOLUMIO_FE_VERSION="e2ca9040ba05aea0dd1b326f29b87c86c5061952"
VOLUMIO_BE_VERSION="b994356475963dd9c244e0e56678e49304670f11"
VOLUMIO_ARCH="arm"
VOLUMIO_VARIANT="volumio"
VOLUMIO_TEST="FALSE"
VOLUMIO_BUILD_DATE="Sun Dec 30 22:53:16 CET 2018"

VOLUMIO_VERSION="2.522"
VOLUMIO_HARDWARE="pi"
VOLUMIO_HASH="d1bd47f832048be085b808eb790728e5"

config.txt:
initramfs volumio.initrd
gpu_mem=32
max_usb_current=1
dtparam=audio=on
audio_pwm_mode=2
dtparam=i2c_arm=on
disable_splash=1
hdmi_force_hotplug=1

Volumio i2s setting below: do not alter

dtoverlay=hifiberry-dacplus

A happy user of Volumio,