Weak Gold Strike?

It does appear to be a weak strike but not being an expert I'm just guessing

The place to look for with weak strikes regarding Mackennal busts is in the area of the band on the orb atop of the crown, and detail in the King's beard.
The coin pictured is not weakly struck.

Gold is a soft metal, and so strikes up fairly easily, with less striking pressure.
Gold coins generally are softer, and so are more sensitive to damage and wear. The main reason why they are more frequently found in higher grades is that gold coins circulated less frequently. In many countries from the mid 19th century and later, gold coin circulation was complimentry with paper money, of equivalent face value denominations.