mid side recording question

I have not much experience with michrophones, I only did vocal recordings,
Now I have a project were I want to record a guitar/vocalist acoustic.

I have a rodeNTK, Stagg condenser and a audiotechnica ATM25

What I did was set up the rode as mid, the stagg as side in a figure of 8 and the ATM25 as an xtra to the guitar for definition.
I found a tutorial on mid side and I think I did it right.
I routed the stagg (the side mike) to two channels, panned them hard left and right and flipped the phase
of one of them.

here’s my problem, lets take the ATM25 out of the loop , just the mid side configuration.
I think it works but when I raise the side channels in equal amounts the vocal drifts to one side.
is this a result of the phase difference, because I hoped that the vocals would stay dead centre and a stereo spread would occur along side in equal amounts…
To remedy this is to pan the mid michrophone untill it’s dead centre again, is this the right way??

Greetz Dylan.