It would also be great to cover this issue on your official resources. In particular, add a description of the problem with the drop in volume in the FAQ, for example, here:
https://www.fiio.com/newsinfo/779363.html, and indicate that a firmware update is required to solve this problem, and that you need to contact to technical support. It is also worth adding K7 to the general list on this page:
https://forum.fiio.com/firmwareDown.do and also write that there may be a problem with a drop in volume and give contacts for contacting technical support.
I found two reviews from people who bought the K7 from your official store on AliExpress and complained about the problem with the volume drop, and these people do not know how to solve this problem. Therefore, I repeat, you should consecrate this problem on your official resources.
Once again. Most importantly,
@FiiO, please check that this rare problem does not occur in your future devices, in particular the K7 BT. Test it well before you start selling.