key value being replaced by 'key' when using merge() in twig

You need to wrap your variable with parenthesis to be able to use it as a key.

{% set params = params | merge({ (key) : value}) %}

With numeric keys you can lose your key in the process, with the m̀erge`filter.

I couldn't find any documentation about the '+' operator applied with arrays, but it works well in this case:

{% set array = {(1): 2} + array %}

Source: https://github.com/twigphp/Twig/issues/2741#issuecomment-417445042