This article explains skipping more than 2 major releases can cause any mis-behaviour post upgradation.

During upgrade even though no any error message or upgradation is done successfully, it can lead to mis-behaviour like interface not coming up, protocols not working as expected.

As per Juniper standards you can skip no more than two releases when upgrading (or downgrading). That means you can upgrade only to one of the three releases subsequent to your current release. If you want to upgrade across more releases than this, you need to perform multiple upgrades.