All credits for finding the root of the evil go to Andrew Hutchinson 
His original e-mail:

There is a bug!

Backend (avr part) creates special "pm()" annotation by looking at RTL. This is done in avr.c by this function:

static bool
avr_assemble_integer (rtx x, unsigned int size, int aligned_p)
{
debug_rtx(x); /*ADDED TO DEBUG*/
fprintf(stderr,"size=%d align=%d\n\n",size,aligned_p); /*ADDED TO DEBUG*/
 if (size == POINTER_SIZE / BITS_PER_UNIT && aligned_p
     && ((GET_CODE (x) == SYMBOL_REF && SYMBOL_REF_FUNCTION_P (x))
     || GET_CODE (x) == LABEL_REF))
   {
     fputs ("\t.word\tpm(", asm_out_file);
     output_addr_const (asm_out_file, x);
     fputs (")\n", asm_out_file);
     return true;
   }
 return default_assemble_integer (x, size, aligned_p);
}

(I added 2 lines to debug it)

You will see it looks symbol_ref or label_ref - otherwise it does not use pm.

So I ran it with following testcase to see what argument get sent:

//Dummy func
int table[]= {1,2};
char ctable[]= {3,4};
void foo(void) {}
 
//Table with address manipulation
void (* const pFuncTable[]) (void) = {
  foo + 0,
  (foo + 1),
  foo +2
};

....and we get the following:

(const_int 1 [0x1])
size=2 align=1

(const_int 2 [0x2])
size=2 align=1

(const_int 3 [0x3])
size=1 align=1

(const_int 4 [0x4])
size=1 align=1

(symbol_ref:HI ("foo") [flags 0x3] <function_decl 0x7fdcf030 foo>)
size=2 align=1

(const:HI (plus:HI (symbol_ref:HI ("foo") [flags 0x3] <function_decl 0x7fdcf030 foo>)
       (const_int 1 [0x1])))
size=2 align=1

(const:HI (plus:HI (symbol_ref:HI ("foo") [flags 0x3] <function_decl 0x7fdcf030 foo>)
       (const_int 2 [0x2])))
size=2 align=1

You will see last two cases are an expression with outer code as const:HI so it will not match, it does not add pm().