The Abbasid Boot Camp technology (+15% to infantry health) seems to overwrite the Elite Tactics upgrade (+10% health to Melee Infantry).
Using an Elite Man-at-Arms as an example, base health is 180. If Elite Tactics is researched first, health increases to 198, which is correct. If Boot Camp were to stack, health should increase to 227.7 (1801.11.15). If Boot Camp increase does not stack, and instead adds 15% of base health, final HP should be 225 (180+18+27).
Currently, once Boot Camp is research, Elite Man-at-Arms health is set to 207 (180*1.15). This suggests that Boot Camp is applied, but Elite Tactics is overwritten.