Just like the title says Clear Absinthe only stacks to 20. It also only stacks to 20 in the Tradeskill Depot
Not a bug. Ngreth has stated that he does not want stacks to 1000 for items with stats or effects even if used in other tradeskill recipes. You can request a reexamination for it, but it’s not a bug.
Agree with Brickhaus about general inventory - they are only meant to stack to 20 intentionally. They both stack over 20 for me in the TS Depot. So does Chronal Absinthe.