Warning: Illegal string offset 'center' in ProximityFilter->acceptExposedInput()

I am seeing the same thing. Have tried changing it to a single filter and then back to grouped but it still has the same behaviour.

The problem appears to be in here:

public function acceptExposedInput($input): bool {
  parent::acceptExposedInput($input);

  $this->value['center'] = [];

  $identifier = $this->options['expose']['identifier'];

  if (!empty($input[$identifier . '_center'])) {
    $this->value['center'] = $input[$identifier . '_center'];
  }

  return TRUE;
}

$this->value is "3" (a string) which presumably corresponds to the option I chose in the grouped filter. Changing the string value with array syntax causes an error and results in bizarre things happening (it changes into another string "A" but xdebug seems to think it is a reference to a class).

This patch fixes the problem.