It’s working not exactly like @SGC have described (you can check your audit history to see how often it’s audited: I would love a way to see when downtime events are recorded - #2 by BrightSilence), the main reason for quick drop in online score that your node is new, it has too low audits number and worked less than a month, but the online score is calculated for the last 30 days.
So, missing one audit from three would mean 33% online score loss.
See How is the online score calculated? - Node Operator
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