He can't be accessed at night.
If you go to the second floor, go into what was your room, go outside to the balcony, climb the vine, and peep in the window you will find what he is up to though.
A big thanks to @BaraGamer535 for their help on identifying the weaknesses for the new characters for this update! Fair warning: don't access this if you don't want spoilers for who is behind the curse!
Note: I am still working my way through the story myself, so I will update battle drops as I finish them, but please be patient with me as I need to grind for the One Ring.
So I managed to get to the half-demon and lost that fight horribly. Advanced the day to evening and now Astaroth's work is available. Since I don't cloud save and don't have the game installed elsewhere I can't be certain if I experienced a one time issue in 9.1 or if it was caused by the fact that Astaroth was technically helping me up the mountain and now no longer is since I defeated the three knights and have the hammer.
I did not as I originally went there in the evening and had not done any work for him as of yet, I assumed it was something that would exist regardless of the day. Let me finish the fight sequences I am in and I will report back. Though I am currently about to fight with the half-demon, so I am not sure if that is going to change how the bug is presenting or not. Give me a few minutes and I will get back to you.
Good morning/day/evening,
First - thank you for the new release! Your hard work is appreciated!
Second - I am not sure if this a bug or intentional but figured it was worth mentioning: I went to Solitude Mountain and proceeded to get my butt kicked. So I decided to go back and grind for more black ore. Went to see Astaroth in the evening, but it will not allow me to work for him just duel. Same problem at night. Is this intentional?
Thanks to @Urosh Uchicha for the idea on formatting a guide in this fashion. Dropping it in a Google Sheet so that I can update it when they make changes to the game. If you find that I have missed anything or got something wrong, please feel free to leave a comment below and I will get it taken care of as quick as I can. Thanks.