I don't understand why it would not be a bug, if it clearly does not match the expected result. See the following definitions:
defect: a shortcoming, fault, or imperfection
A software bug is an error, flaw or fault in a computer program or system that causes it to produce an incorrect or unexpected result, or to behave in unintended ways
Even if the solution is not a code change, it still should be researched, and actions could be taken. The test data and the model are also part of the computer system.
I always report unexpected degradation in performance or accuracy. Again you probably need to research it and decide if this the new acceptable baseline.
Should I create a task for the developer to improve his model?
Do you represent the stakeholders/business? Does the developer? If yes, decide if the model needs improvement. If no, consult the person(s) that use this feature. Do you have enough data to assess the risks? Don't make quick assumptions.
Reporting defects is not about right or wrong, following the specs or not, but about does this still adds the right value to our product, business, etc..
Keep these modern testing principles in mind:
- Our priority is improving the business
- We believe that the customer is the only one capable to judge and evaluate the quality of our product
- We use data extensively to deeply understand customer usage and then close the gaps between product hypotheses and business impact.