It’s possible that the $context already contains an ACF object with the key acf and your code overwrites it. This will then destroy any subsequent use of $context['acf'] which Timber (or your Theme) needs.
Try using a different key in your own code ($context['my_acf']) and see whether that helps, or just do a var_dump of $context['acf'] before the first line of code in your example, to see whether it already exists. Timber objects/context already have a lot of data in them, which you might not know already exists.
