Ethernet: distance from PHY to magnetics
First purpose of the magnetics on PHY is create a BALUN ( or interface BALanced line to UNbalanced IC and visa versa) This improves the Common Mode Rejection Ratio CMRR significantly over  the full signal  bandwidth.
Secondary requirements are for impedance matching.
Third requirements are by improving CMRR is to reduce radiated CM noise.
Fourth is provide immunity to expected EM fields , ESD etc.


When stray common mode magnetic fields are coupled to nearby unbalanced lines, it defeats the purpose. Due to inverse square law, couple after about twice the size of the magnetic core may be sufficient to achieve adequate CMRR but being unbalanced signal and ground impedances, making the path long exposes it to other sources of noise than be converted from CM to differential mode due to differences in coupling of different impedances.
Magnetic core in 100MHz range and up tends to be conductive ceramic mix and also susceptible to conductive coupling of ESD as opposed to the more insulative LF high mu ferrite cores.