Regretfully, it seems your numbers from testing stacking was not correct. After extensive testing, I can confirm that like all other mastercrafts, X% Damage per Nearby Enemy is not an exception. It does not stack when identical mastercrafts are in use, but does stack when similar mastercrafts of different tiers are used.
Test results quoted from my thread:
+5% Damage for Every Nearby Enemy (Mastercraft)
After a user reported that 7.5% damage for every nearby enemy supposedly stacks, I decided to retest some mastercrafts, including this one, to see if it's just a failure of UI, but not failure of functionality. Both +15 Stamina and +3 Guard/hit still do not stack at all.
Setup 1: Level 19 warrior, weapon damage 91, base 4% attack.
Setup 2: Level 19 warrior, weapon damage 91, base 4% attack, 5% Dmg/Nearby Mastercraft x1.
Setup 3: Level 19 warrior, weapon damage 91, base 4% attack, 5% Dmg/Nearby Mastercraft x3.
Target: Level 11 Bear
Test: test whether the mastercraft stacks against a single target when no other enemy can even be spotted nearby. 10 regular hits with each setup, crits are ignored.
Results (expected vs actual mean):
Setup 1: 47.84; 48.8
Setup 2: 52.57; 51.8
Setup 3: 62.04; 52.1
Conclusion:
The increase in damage between setup 1 and 2 fall almost perfectly within the expected values. However, Setup 3's result falls far too short of what can be expected from a 15% base damage increase. It seems that at least the 5% Damage for Nearby Enemy mastercraft does not stack.
+7.5% Damage for Every Nearby Enemy (Mastercraft)
To remove all doubt, I went and farmed some of FT Bloodstone for the test. I also opted to use a weapon with higher weapon damage, so that the results may be more clear.
Setup 1: Level 19 warrior, weapon damage 147, 43% attack.
Setup 2: Level 19 warrior, weapon damage 147, 43% attack, 7.5% Dmg/Nearby Mastercraft x1.
Setup 3: Level 19 warrior, weapon damage 147, 43% attack, 7.5% Dmg/Nearby Mastercraft x3.
Results (expected vs actual mean):
Setup 1: 145.86; 145.6
Setup 2: 161.63; 156.6
Setup 3: 193.16; 155.1
Conclusion:
It's clear that the same type of mastercraft would work the same, regardless of tier. While the increase between setup 1 and setup 2 is expected, there is no increase between setup 2 and setup 3.
My guess is that any difference is either the result of RNG, or while you were testing, a trolling Ram or Fennec ran past or stayed too close. Those neutral animals contribute to the mastercraft's damage bonus.
This particular type of mastercraft is still one of the best in the game for a melee DPS, but there is no point stacking it unless they are from different tiers. i.e. 5%, 7.5% and 10% on three different pieces. Alternatively, you can use some of the other mastercraft slots for other useful effects.