I installed qoi-ci-1.1.1 plugin for all supported programs.
XnViewMP, using both Save As and Batch Convert, always saves RGB 24-bit QOI images from 24/32-bit sources.
No options/settings were possible to change in saving.
No image modification actions were used in batch conversion.
Also, conversion fails from paletted 8-bit PNG images (and possibly other limited formats).
It skips such files, which may be overlooked in large batch.
User must explicitly add changing color depth to 24/32 to succeed.
I think I had this problem not only with saving QOI, not sure.
It may be XnViewMP's own bug, but worth mentioning anyway.
Paint.net and GIMP are OK with the same plugin version 1.1.1.
Saved image is always 32-bit with RGBA header mark, regardless of alpha content and without possible user choice.
Which is fine, at least being lossless and nothing to waste (unlike BMP/PNG with alpha channel).