Greece- requirement to change "user" every 6 month on 18 month transit log?

[SY_Maria]

I am non EU citizen and own US-flagged boat with EU VAT not paid. I skipper the boat myself. We entered Greece last year and were given transit log that expires in 18 month. Now I have an agent telling me that unless I change "user of the boat" every 6 month, it is a violation and heavily fined.

Is there is a rule like this?