Upgrading Frostfang reduces your mp5

Spellthief's edge gives you +25% base mp5. Upgrading it into Frostfang increases the bonus to +50% base mp5 but upgrading Frostfang into Remnants of the Watchers reduces that amount back to +25% base mp5. I'm quite positive this is not intentional, as you don't typically lose stats as you spend more gold on items.