I don't know if the team knows this, but I haven't seen a bug report on it.
Cities with the ability to build level 3(or 4 if you count clearings) trade ports, do not have the ability to build level 2 level trade ports. This, of course, prevents them from getting level 3 ports unless they start with level 2. The problem is, cities that should be able to get level 3 need both hidden resources "tradeport" and "bigport". They only have "big port" currently.
Sinope is the exception, for some reason it already had both resources.
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