I tried 3 different browsers and the app, nothing works, the dropdown menu of Spell Mastery (level 18 Wizard) is blank and show no options/spells to select. 

Quote from LegendmakerGM >>

I tried 3 different browsers and the app, nothing works, the dropdown menu of Spell Mastery (level 18 Wizard) is blank and show no options/spells to select. 

 
Are there any 1st- & 2nd-level spells in your character’s spellbook?

I have the same issue, I do have all levels of spells in my spellbook but I can't see anything in the dropdown menu :( 

I’m also having this issue. 

Please post the URLs for your affected characters so someone can look into this issue.

Yes, there were level 1 and 2 spells.

To replicate just make, for example, a Bladesinger wizard level 18 and try to select the dropdown for spell mastery which is granted at level 18

That's what I did on my end and the dropdown presented no options to select from

https://www.dndbeyond.com/characters/107558226

Mine is doing this as well. I was able to select the spells but now I am no longer able to change them. https://www.dndbeyond.com/characters/129526442

Same problem. I had spells selected and they were still showing, but the dropdowns were empty. I reduced the wizard's level to 17 and re-added level 18 as an experiment - the spells cleared, but there are still none selectable in the dropdown

Confirmed, I'm seeing the same bug.

Drop downs for Spell Mastery and Signature Spells don't work. 

Same issue, both with Homebrew spells linked directly to a homebrew subclass, spells from partnered sources, such as the Book of Ebon Tide and Tales from the Shadows (Even while using the subclass that came with that book). And basic spells from the base game, such as Fear, Cause Fear, Counterspell, Shadow Blade.

I tested a basic, non-homebrew wizard, no external books, no legacy features, and no subclass. Both features are completely dead, doesn't matter if the spell is prepared or not..

This still not working after all the changes 

I can't select my spell mastery spells 😭😭😭😭😭😭😭😭😭😭😭😭

Having the same issue - have tried both legacy and 5.5e versions of spells and neither appear in the dropdowns

Same here, I even tried getting rid of all spells except for one level 1 and one level 2 and it still shows nothing. Thought by potentially making it easy for the app but still gave nothing. 

Still not working guys...

Having the same issue with this....

The 5.5e versions of the features work, but the 'repeatable' ASI bonus feat doesn't work (Can only be grabbed once). So no point in making a 5.5e wizard either.

I was trying to make a 2014 Wizard to test something and was faced with the same problem; I can't choose any Mastery or Signature spell either.

Same issue for me as well.