It was just an input-bug

Japt, 22 21 bytes

¬Å£¬qYîÃy w ê y w ê ·

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Explanation

The first 8 bytes generate the bottom-right quadrant of the pattern:

 ¬ Å  £    ¬ qYîÃ
Uq s1 mXY{Uq qYî} 

Uq                 : Split the input into characters.
   s1              : Slice off the first.
      mXY{      }  : Map each item X and index Y to
          Uq       :   the input split into chars,
             q     :   joined with
              Yî   :     Y spaces.

At this point we have an array of e.g. ["ABCD", "A B C D", "A B C D"]. Unfortunately, it takes 13 bytes to square this:

y w ê y w ê ·
y w ê y w ê qR
y                : Pad each line to the same length with spaces and transpose.
  w              : Reverse the array, and
    ê            : palindromize. Now we have the bottom half of the output transposed.
      y          : Transpose back.
        w ê      : Reverse and palindromize again, giving the full output.
            qR   : Join with newlines.
                 : Implicit: output result of last expression

Python 3, 149 141 95 bytes

def f(s):l=len(s)-1;[print((' '*abs(i)).join(s[:0:-1]+s).center(2*l*l+1))for i in range(1-l,l)]

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Thanks to @KevinCruijssen and @ETHproductions for saving some bytes

Special thanks to @notjagan for saving 46 bytes!

05AB1E, 17 bytes

g<F¹RSðN×ýû})Rû.c

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-1 thanks to kalsowerus.