According to Saintone's Thread,
Quote
Refinement % on many other gears (e.g. Vicarious Calling weapon) have been scrambled as well, and it's strictly a visual change. You can refine these items, you just won't know what % they were previously unless you memorized them before the maintenance.
I thought it was safe, as it was supposedly a visual change.
Being curious if I could enhance my +8 weapon (previously roughtly 93% with 7650~ atk) to a "true 100%" (around 7760 atk), I decided to reroll my guitar.
Example of a "real 100%"

However after rerolling my own weapon, this is the result:


It does not go any higher from the nearly 100 times I tried enhancing.
Considering my guitar had around 7650 attack before, i just lost nearly 200 attack that I can never regain, not to mention the 30m+ on refiners and nearly 200k aethar that was spent on this...
I don't know if this is fixable, but it's definitely something to be wary of.