box.auto_increment counter resets

When inserting more than 256 keys with box.auto_increment it inserts tuple with repeated first key.
Problem seems to be in index:max() op, that simply takes rightest leaf in the tree.
Problem can be reproduced with:
for i = 1, 1024 do
    box.insert(0, i, i)
    print(box.space[0].index[0].idx:max())
end
Output is:
---
(...)
251: {251}
252: {252}
253: {253}
254: {254}
255: {255}
255: {255}
255: {255}
(...)
255: {255}
255: {255}
511: {511}
511: {511}
(...)
---