If you've already seen this task somewhere, I'd be grateful for the link.

You are given a string of length about $$$10^5$$$. There are many queries about intervals of this string and for each interval, you have to calculate the number of distinct subsequences (not necessarily contiguous) of this interval.

The solution builds a segment tree and keeps matrices in each node. Have you seen this problem?

cs academy task subsequence queries Link

Great, thanks! If somebody has seen this task in any other place, more links would also be useful.

Here

A slightly different problem but a similar type(DP + segment tree with matrix in a node) https://codeforces.com/contest/750/problem/E (By errichto).

Do you mean problem K, Subsequence Queries from XVIII Open Cup, GP of Korea? The same statement with the problem of CSAcademy, but with constraints $$$N,Q\leq 10^6$$$ and $$$\vert \Sigma \vert =26$$$.

Link to the contest: XVIII Open Cup named after E.V. Pankratiev. Grand Prix of Korea.