Ok. I know some of the services don’t work on a 1-100 volume scale, but on 1-10 or 1-20 scale, and thus volume changes are rounded up/down to accomodate.
In fact, I would guess that maybe that’s why. If you have Alexa installed, it may be that Alexa accesses the volume data, just pinging the speaker periodically, and sort of forces it to level 5, as 2 doesn’t make sense to it, but 5 translates to 1 on Alexa’s volume scale. Something like that. I have tested this, but also haven’t been concerned about low level volume.
Either way, it would probably be a good idea to create a diagnostic next time this happens. You can contact Sonos support and with the diagnostic, they should be able to tell you what’s happening to adjust the volume...whether it be Alexa or something else.