See code below. rdmd main.d fails with 'Error 42 Undefined Symbol'.
Unexpectedly,removing the comment around import B in main removes issue at cost of widening scope.

// main
void main() {
   import A;
// import B;
   import std.stdio;

   writeln("Entered main");

   fnA1();
   
   writeln("Leaving main");
}

module A;

void fnA1() {

   import B;
   import std.stdio;
   
   writeln("Entered fnA1");
   fnB1();
   writeln("Leaving fnA1");
}

module B;

void fnB1() {
   
   import std.stdio;
   
   writeln("Entered fnB1");
   writeln("Leaving fnB1");   
}