When titans replenish, they go back to max stack size. Has nothing to do with durability. They do have a hidden durability mod also. Non etheral titans must lose 6 durability points over 4 seconds in meele (the amount of time it takes to replenish 1 jav) in order to break one. Your max stack will drop from 140 to 139 but a second later it will replenish bringing stack back to 140. Etheral are different. Durability is only 4 on each thus you must lose 4 durability in under 4 seconds in order to break one. This is very hard to do but I guess could be done with a high ias impale or something. It makes sense to me that if you were at 100 javs when you broke one (before replenish took it to 101) that overtime you would still get back to 140. I have heard of several people breaking one and max stack perminately drops to 139 on etherals. My theory is that this can only happen if you were at 140 stack size when you broke one. I could be wrong on this. Maybe you could be at less than max stack and break one thus perminately reducing max stack. I used to meele all the time with my etheral titans and never reduced max stack. We did some math on this a while ago and calculated that you would have to jab for about 45 minutes at 5 frames in order to guarantee 1 etheral jav breaking before the replenish kicked in. Very rare.
Either way, any etheral javs/throwing weapons without the replenish mod are worthless as they will break/run out very soon.