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Anything between '[' and ']' is evaluated as a math expression. I will explain each command one by one :

'echo ${r[abc]}':

Here 'abc' will be replaced with 0, so it returns the 0th element from the array.

echo ${r[---abc]}:

Here math expression is evaluated as '-(--abc)'. Since 'abc' is 0, it will evaluate to '-(-1)' -> '1'. So it returns 1st element from array.

echo ${r[---abc]}:

It's the same as previous expression, but 'abc' is set to '-1' due to previous command. It will evaluate to '-(-2)' -> '2'. It returns empty element because array does not have a second element.

This is not a bug in bash.