\prg_new_protected_conditional:Nnn  % warning on this line
  \example_expandable_first:
  { T, F, TF }
  { \prg_return_true: }
\prg_new_conditional:Nnn
  \example_expandable_second:
  { T, F, TF }
  { \prg_return_true: }
\prg_new_protected_conditional:Nnn  % warning on this line
  \example_expandable_third:
  { T, F, TF }
  {
    \example_expandable_second:TF
      { \prg_return_true: }
      { \prg_return_false: }
  }