It it probably more correct to refer to the socket as a Tip Ring Sleeve (TRS) socket, rather than stereo. The guitar plug is a Tip Sleeve (TS) plug.
When you plug in, the Tip part of the plug initially engages with the Ring part of the socket. Push the plug further, and the Tip components of the plug and the socket will connect. If you have soldered the output wire from the guitar to the Ring connector, this would explain the issue you are having.
In the barrel style of socket, it is practically impossible to visualize what is going on, but this image may help. You can use a TS plug in a TRS socket by leaving the socket's Ring connection unattached.
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