Local currently sets the value of serialize_precision in php.ini.hbs to be 17. This was causing some floating point precision errors in my WordPress REST API, when running on Local.
Starting from PHP 7.1.0, the default value is now -1. Manually changing this value in my php.ini.hbs to -1 fixed the issue I was encountering.
I think Local should update this value to be -1 by default.
Source: PHP: Description of core php.ini directives - Manual