Join on QuantityArray

Question

I need to merge two QuantityArray objects of mixed type. My first attempt looks something like:

q1 = QuantityArray[RandomReal[{-10, 10}, {10000, 4}], "Angstroms"];
q2 = QuantityArray[RandomReal[{-10, 10}, {10000, 3}], "Debyes"];
qnew = Join[q1, q2, 2]; // AbsoluteTiming // First

0.167756

But it turns out this builds a List of List of Quantity objects:

qnew[[;; 3]]

{{Quantity[4.1543, "Angstroms"], Quantity[5.47902, "Angstroms"], 
  Quantity[-2.40716, "Angstroms"], Quantity[-6.48639, "Angstroms"], 
  Quantity[1.23, "Debyes"], Quantity[6.39028, "Debyes"], 
  Quantity[-3.68544, "Debyes"]}, {Quantity[-4.16621, "Angstroms"], 
  Quantity[3.38214, "Angstroms"], Quantity[-6.28603, "Angstroms"], 
  Quantity[-8.62747, "Angstroms"], Quantity[-2.42187, "Debyes"], 
  Quantity[5.7411, "Debyes"], 
  Quantity[-7.45621, "Debyes"]}, {Quantity[1.04501, "Angstroms"], 
  Quantity[8.00975, "Angstroms"], Quantity[-6.73537, "Angstroms"], 
  Quantity[9.31479, "Angstroms"], Quantity[-3.87661, "Debyes"], 
  Quantity[6.69572, "Debyes"], Quantity[4.10434, "Debyes"]}}

It's also super slow as that 10000 becomes closer to 50000.

I found that this is faster and builds a proper QuantityArray:

qnew =
    QuantityArray[
     Join[QuantityMagnitude@q1, QuantityMagnitude@q2, 2], 
     Join[First@QuantityUnit@q1, First@QuantityUnit@q2]
     ]; // AbsoluteTiming // First

0.006938

qnew[[;; 3]]

Likely as the packed arrays never unpack as they would in the first case.

Now the question is that I was wondering if there was a reason for this behavior. I.e. is there something that makes the first behavior of Join more general than the second?

Otherwise I'm tempted to forward this to WRI as a suggestion.