PoE2 Expedition vendor item level cap 76-78 with occasional 79.

Why is the expedition vendor item level not increasing after some point?

I am level 87 and at T15 maps.

The highest lvl item l have seen is lvl 77 and item levels are mostly stuck at 76-78 and 0-3x ilvl 79 items appear per roll.

I am playing grenade merc and it is pretty much impossible for me to get even a single siege crossbow from them, because they start appearing at ilvl 79.. This makes the weapon merchant useless for me.

Not sure for POE2. In POE1 they continue to increase with character level, but more slowly at higher levels. I don't get lvl 86 items regularly until i'm level 91. For the early levels you tend to get items higher than you are. I think there is a scale that it works off, but I have never seen exact numbers.