Could be normal fissuring of the bark as it ages. I know mature lilacs aren’t exactly smooth barked

Just checked my 200 ft. of common lilac hedge and a couple of 'stand alone' plants. I cannot see anything that looks like your picture. There are the horizonal lenticel distributed along the stems but no vertical slits in the bark.

If you have an agricultural extension office nearby, you could show them the pictures. If they don't know, they could find out. You may have to bring them a sample stick for them to diagnose.

Erm, not any sort of tree expert here but I just wondered if , when you planted it, you fertilised it? Or a nitrogen bump this spring? A mad growth spurt?.

Thank you for your interest in the topic. Especially for checking 200 ft of hedge :) I'v checked my neighbour's hedge too (no cracks).

Maybe they had nitrogen bump but cracks occurs only on one stem on plant with 2 stems (one plant is completely cracks free, one has few).

I'v sliced the most cracked stem a bit.

Looks like cracks are not interconnected inside deeper layers of wood. I think Crosssection also looks good.

I was a little scared because from the outside it looks similar to fungus Anisogramma anomala (Anisogramma anomala's host is only the European hazelnut), so I thought it was some kind of fungus, but since the cracks are not interconnected, maybe there is some defect related to uneven development or or some kind of nutritions deficiency.

TBH, I didn't check the whole 200 ft. ;-)

But here's a link that you could check out:

https://www.g-fras.org/en/world-wide-extension-study/europe/eastern-europe/poland.html#extension-providers

That's a good point, nel, and explains why the stems look healthy inside.

i'm not sure the stems look healthy inside. There's a brown staining and some brown spots. I'm thinking something fungal. Hail wouldn't hit only some of the stems.