When you activate Fend, you get the better spear guard status effect. As usual, this lasts for two turns.
If you attack someone with your spear on the same round that you use Fend, then the boosted spear guard effect stays.
However, if you attack with your spear on the next turn (and you probably need to do this), then the spear guard effect is reset from 1 to 2 turns, and in the process is downgraded to "normal" spear guard block amount.
This means that for practical purposes, the good block amount from Fend only lasts for one enemy turn. A bug, an oversight, a technical limitation or WAD?
In any case it devalues an already niche feat, so it might use a tweak.