An 0805 resistor is typically rated 150V with 200V overload, but arcs across the ~1.2mm pad spacing above this, at a few kV.
An 0603 resistor is typically rated 50V with 100V overload, but arcs across the ~0.9mm pad spacing above this, at a few kV.
You would need more resistors with larger spacings for the string. MELF parts like
SMM 0204 are >2kV pulse rated with 1.7mm pad spacing, so at least three are used in a series string.
These DMM's have a terrible PCB layout, it's student-grade. It's not only the resistor's spacings.
HV jumps between the PCB traces to the ground pour, or to the battery holder, or across the rotary switch, to the fuse holder etc.
The PTC is a mile away from the input jack. They just don't have spacings and clearance required.
Schematic J1-A (PTC path) is (output) for ohms, capacitance, diode test etc.
J1-B (900k ohm path ) is to a comparator for continuity beeper and frequency I think.
The A/D input is after the 10MEG resistor.