I am the author of this question, Bogumił Kamiński helped me solve the problem, however, I changed computers, I am using MacOs and when I went to reproduce the example, I couldn't.
this is my code in the julia environment:
julia> setprecision(BigFloat, 20)
20
julia> x = big"1.23456789"
1.0
should be 1.2345676 and not 1.0.
FYI, I was able to fix this by installing Julia via juliaup rather than directly through Homebrew. It seems there's an issue with the Julia builds published to Homebrew, I've opened an issue regarding this though the repository maintainers have closed it. Maybe you can leave a comment saying you're experiencing the same thing: https://github.com/Homebrew/homebrew-core/issues/131422#issuecomment-1555071590