Aye same thing happens to me where I have my points then some matches it says I have zero nada nothing. They come back eventually which is good I guess.

The favour tracking is schizophrenic at best. Only thing you can do is buy the item you want before your favour resets. Here's what happens:

I unlock Swift Anklet with 450 favour points.

I buy Swift Anklet in-game for 450 gold (pretend numbers).

Swift Anklet is now usable for any demi in any multiplayer game. Singleplayer and multiplayer favour are not connected.

When your favour resets to 0, you should still be able to equip the item your purchased, though all other items will be highlighted red as normal.