I am actually working with this sort of thing right now. This has been working for me:
I think the main difference is using the well to store the data. I remember that I also ran into issues using the labware itself, but I could have been making some mistake.
EDIT: See this thread and the threads linked: Custom liquid class with tunable zOffset - #2 by Snowball
EDIT 2: My example here actually has a bug in that I thought I was writing/reading from an array attribute, but it seems like that worked without actually creating an array. It just creates a well attribute under the key “commands[1]” which is not exactly what I intended but works. Just wanted to include that in case someone in the future tries to copy it.