When using vertical bars | in a table in the live preview double bars appear.
Creating a new table and putting |x| in one of the cells (which Obsidian will helpfully automatically turn into $\|x\|$ renders with double bars:
Workaround: use $\vert x \vert$.
There are some similar bug reports, but none that quite match this behaviour or are recent enough to include the live-preview table editor (as far as I can tell)
This is problematic because it makes discerning norms and absolute values difficult. I am able to reproduce this in the Sandbox Vault on Obsidian 1.8.9