MCP3461 analog side broken "somehow"?
Your setup might look something like this.  The cable capacitance depends on the gap of the conductor pair but is typically 25 to 35 pF/m and the rise time of the 1st pulse is determined by the pullup current.  Since this shared bus has asymmetric impedances 4k7 for hi and &lt; 75 Ohms for low the fall time is much faster.
There are 2 sources of potential problems.

The added length to the LCD terminal may add too much capacitance for 4k7 pullup at the clock speed you are using.  Reduce the pullup R by adding 4k7 in parallel then 1k to see if there is any improvement.

if using SMPS for both devices and now sharing a common 0V gnd, there is a high probability that the RF leakage in the SMPS transformer which isolates DC is now injecting noise currents on your ground signal.  This can degrade the 1st bit as it switches from tristate to active state.  This may require an RF cap from AC gnd to DC gnd to suppress, if this is the case or STP cable and a detailed layout (photo) and signal capture on a DSO to analyze root cause.

Possible 3rd source is common mode noise coupling into the cable from the SMPS or line f that degrades the signal integrity to make it marginal on the 1st bit or byte.