What happens if you put a piece of copper into a solution containing Al3+ ions?
You are right with copper does not react with aluminium ions.

But as a side reaction, copper may get slowly oxidized by oxygen and dissolve in mildly acidic solution of aluminium salt. 

Such a thing may happen, if you decalcify copper heating spiral by vinegar and let it stay overnight.

$$\ce{ 2 Cu + 4 CH3COOH + O2 -&gt; 2 (CH3COO)2Cu  + 2H2O  }$$

Something similar can happen in aluminium salt solutions as well, because of hydrolysis.

$$\begin{align}
\ce{ [Al(H2O)6]^3+ + H2O &amp;&lt;=&gt; [Al(H2O)5(OH)]^2+ + H+}  \\
\ce{ 2 Cu  + O2 + 4 H+ &amp;-&gt;  2 Cu^2+ + 2 H2O} \\
\ce{ 2 Cu  + O2 + 2 H2O &amp;-&gt;  2 Cu(OH)2 v} \\
\ce{ 2 Cu  + O2 +  H2O + CO2 &amp;-&gt;   Cu(OH)2\cdot CuCO3 v} \\
\end{align}$$