When I bleed and refill I wait until the fan cycles twice before I put the caps on. It can idle all day and never reach a half on the gauge.
Incorrect fill procedure. You probably have a pocket of air that is moving through the system. As the system cools it will give back/take coolant from the res.
Fill it up (slowly, you can also squeeze the upper and lower hoses to burp/cause a siphon). Run it until the fans cycle. Let it cool. Check level again and add as needed. Repeat.
This is why we use a very expensive Snap-On tool at the shop to fill systmes. I actually pull them into a vacuum pre-filling. When I crack the valve, the vacuum pulls the coolant into the system. No air. EVER.