Ok, I've done a little research and I saw this as an issue earlier in Marlin threads but did not find resolution if there was one.

I have a Tronxy X5S (corexy) with a ramps board, I am using Simplify 3d as my primary slicer.

With a normal compile of Marlin 2.0.3, I can watch the temp log in Simplify3d go to INSANE readings on the bed. *IMPOSSIBLE TEMPS*

Since I like to view the graph for temperature stability this negates that option. I am pretty sure this is a Simplify3D error. But I have a "fix" that I have done in Marlin.

I've not done a lot of coding in Marlin so I wanted to ask if this hack would cause other issues for slicers.

What I discovered was that Simplify3D was at some times display/logging the step count B: as Bed temperature (improper line parsing in Simplify3D most likely)
Looking at stepper.cpp section
#ifdef __AVR__
if (was_enabled) ENABLE_STEPPER_DRIVER_INTERRUPT();
#endif
#if CORE_IS_XY || CORE_IS_XZ || ENABLED(DELTA) || IS_SCARA
// SERIAL_ECHOPAIR(MSG_COUNT_A, pos.x, " B:", pos.y);
// false read testing
SERIAL_ECHOPAIR(MSG_COUNT_A, pos.x, " b:", pos.y);
#else
SERIAL_ECHOPAIR(MSG_COUNT_X, pos.x, " Y:", pos.y);
#endif
#if CORE_IS_XZ || CORE_IS_YZ || ENABLED(DELTA)
SERIAL_ECHOLNPAIR(" C:", pos.z);
#else
SERIAL_ECHOLNPAIR(" Z:", pos.z);
#endif

I changed the " B:" to " b:" (in the line under my comment of false read testing)
This corrected the problem and I do not get any false hits in the temperature plot or display anymore.

Is that value used by any slicer?

If not then, maybe the A and B should be changed to Ax and By for CoreXY and Cz -- this would be a more "readable" value for learning printer people I think (before they read and understand the relationship completely between X and Y axis stepping in COREXY mechanics.

I will be documenting submitting this to Simplify3D as well so that heopfully they can correct there code.

Just a thought for a "quick fix" if there is no other impact potential.