Defense Value for Shields above +10

On my server, using v15 source, any items upgraded above +10 will have increased DEF or ATTACK (up to +20) but it only applies to Weapons and Armor. Whenever a Shield is upgraded +11 or more, it will start to get really long numbers, such as DEF 512735 ~ 694182.

Where in the source or resource can I change this, for Shields?

My best guess is that your expTable.inc doesn't support upgrades beyond +10 and uses uninitialized integers instead. Your "expUpitem" field should have 20 rows.