Its working correct
When a back pack is worn the weigt of the backpack does not count against encumbrance but the items inside do.
A backpack increases you Str for encumbrance only.
A Consumer bp increases 1 Str a Industrial bp by 2 Str.
If you have the Backpck in your inventory and its set to Carry your encumbrance will show the backpack as weight. When you set it to Worn you will see you Encumbrance value decrease and the values of you limits increase 1 or 2 depending on which type you have.
I have it adjusting your limit values for encumbrance not adjusting your Str at all.