Is there a reason why upper eyelid filler is disappearing after a month?

The duration of the filler depends on the type of the filler, the amount injected, the area injected and then your own metabolism.
Upper eyelid hollowness is tricky, not a lot of filler goes there. I suggest you go back to either of the oculoplastic surgeons and seek their advise. Most likely they will try a different filler from the same brand or maybe move to a different brand if they are comfortable or maybe they will inject more filler to see if it will last longer.This is to be discussed with your doctor.

Hello, it is strange that your filler is dissolving so quickly. It may be because of the type of filler that's being injected and it dissolving quickly. That area of the face should not break down so quickly because of minimal movement that high up in the orbit and is not moving so much during eyelid blinking. Or it may be the injection technique as well. Lastly, everyone's metabolism is a bit different and possible that you break down filler quicker, but one month is really too short. Would inquire as to type of filler and see an experienced oculoplastic surgeon who performs this type of specialized injection frequently.

The quality of different fillers has varying durations of effectiveness. Some last longer than others due to their composition and how the body breaks them down. Were similar types of fillers used in both instances?

How quickly a filler breaks down can also depend on individual metabolism and lifestyle factors. Do you have any medical conditions or habits that might affect filler longevity, such as smoking or certain medications?
The injection technique of the injector can significantly impact how long the filler lasts. Were the injections done by experienced oculoplastic injectors with a track record of successful outcomes?
Product Storage and Handling of filler products must be stored and handled correctly to maintain their efficacy.
Sometimes, the natural anatomy of the area being treated can affect how long fillers last. For instance, areas with more movement or thinner skin may metabolize filler more quickly.
If the filler isn't lasting as expected, alternatives could be explored. This might include trying a different type of filler, adjusting injection techniques, or considering complementary procedures that address hollowing without relying solely on fillers.