This is a bit off topic, since it’s really just a fact of all TRS plugs when comparing a stereo unbalanced feed to a mono balanced feed, but I’ll answer it since it seems to be a common question.
The plugs and cables don’t themselves care what signals you put on what pins, what matters is that what you plug between interprets them the same way. In your case: a stereo pair of unbalanced channels (e.g. a headphone out, or a stereo TRS jack, doesn’t matter 1/4" or 1/8") plugged into a mono balanced socket (e.g. a mixer line input) will result in the mixer “hearing” the difference between the two stereo channels, since it is expecting balanced single-channel data where there is in fact equal-polarity two channel content. Conversely, if you plug any TRS cable into a socket expecting a TS (unbalanced mono - like an instrument input or a guitar pedal) you will usually either just ignore one channel (usually the right channel, carried on the ring) or depending on the physical implementation of the socket, you could short that channel to ground - which could damage some outputs (but most are buffered, with enough output impedance to not really care).
In short: know what you’re plugging in to what - phone jacks of all flavours cross over a variety of standards and they are not necessarily compatible despite the physical similarity.
