The most likely reason is in the latest patch notes…
Now, only Conversion will be used to upgrade your Familiars, the Melding option has been removed.
- Conversions guarantee 1 Normal Familiar of equal or better quality.
- The new Normal Familiar created from the Conversion is bound to the player and cannot be sold.
- Any newly summoned or purchased Normal Familiars are not tradeable.
This is an intended change, not a bug.