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The problem is 99% in your cable:
10mm2 and 2.5m long cable is on a weak side, but should be enough, given all other conditions are perfect. Cheap cables have really crappy (thin and weak) clamps and they are the source of problem. That said it also depends on where you connect the cables on both cars. Are you using terminals under the hood or you connect to battery? 2.5m 10mm2 cable itself has a drop of at least about 0.5V at 50A. If you have crappy clamps, easily add 1-2V more. If you connect directly to the battery on both cars, and if it has quite low charge on recipient car, add 1-2V more and there you are, 11.8V...

Now the cable from the alternator to the battery in the trunk is also quite long, so need to take that into account as well, though its much bigger than 10mm2. Terminals under a hood have a shorter path to the alternator, so it should be worth to connect to those clamps at least on donor car. On the recipient car I prefer to connect directly to the battery when using 2 car setup, and charge it for a while until its voltage raises.

Remaining 1% is due to a possible fault in the battery of recipient car which shorts one of cells and then battery itself and then your cable just can't push enough current to keep its voltage. What's the voltage in recipient car when engine is running and when its off, without additional supply?