I'm on PS5. I normally have Shadowheart in my party, so I have never had her disappear/go invisible.
I have seen other users report she will leave your party if:
You complete the Gauntlet of Shar/Nightsong mission without her.
I have also seen a player report she has gone invisible, if she did not follow your party when:
Boarding the boat to go from Underdark to Grymforge

To check if she is still in your party:
[Linked Image from i.ibb.co]
Go to Camp (R2, Down)
Then go to your inventory (R2, Up)
Then press L2, this will show you all current party members you have recruited, including inactive party members.
Check and see if her portrait still appears on the list.