The problem:
when you write a matrix formula that ha sa single cell output, copy it and try to paste it into multiple cells, which a perfetcly acceptable operation, you get a wrong result in the destination cells (a VALUE error).
This is to realize because if you paste it into a single cell, then copy the two cells where you now have the same formula, and paste them in a multiple selection - say the next 10 cells - you get all correct results. This shows the bug, and is also the workaround I am using at the moment to overcome the problem. I must say it is quite embarrassing with my students, however...

Steps to reproduce:
1 - fill A1:E10 with any numbers; now in cell G1 write the formula SUM(A$1:E$1*A2:E2), and enter it as a matrix formula, so to obtain the vector product (e.g. A1*A2+B1*B2+C1*C2+D1*D2+E1*E2). 
2 - Now copy the formula in G1, select G2:G9 and paste it into this range. You'll get 9 #VALUE! errors in the destination cells, while you should obtain {SUM(A$1:E$1*A3:E3)} instead in G2, for instance.
3 - the problem is that the matrix property of the copied formula is incorrectly gone lost!
4 - now copy G1 again and this time paste it in G2 only. The result is correct.
5 - Finally, select G1:g2 and copy it; then paste it into G3:G9, and you'll get the correct formulas in all cells.

This problem did not exist in older versions.
In some ways, this could be linked to bug https://bugs.freedesktop.org/show_bug.cgi?id=83461, another hint that something was recently changed in the matrix functions logic in a not fully functional way.