Omega2S + audio = audio distorted

Dear community
I'm using Openwrt 19.07 + Omega2S as black box for music playback using I2S, and have noticed that music is interrumped every time I run a write to flash command (just few samples, very easy to check using a wave 1KHz tone).

In order to check it I run madplay, or aplay, or mpd player with a 1KHz tone (mp3 or wave format do the work) and simultaneously write to flash using a command like echo "some_text" > /test_file (/test_file is located in root of squash file system).

I've check the log files and dmesg, but no warning/error/underrun is shown. Same issue occurs with HLK-7628 or SWK92A all of them based in MT7628 SOC.

It will be nice if someone can help me to debug further such issue.

BTW, same test with DraginoHE board and openwrt 14.07 work flawlessly

Best regards,
Sergio