Computers are bad at periodic numbers. The ratio of 8/56 (or 1/7) is periodic (0.142857142857142857...) and thus cannot be stored perfectly. As a result, the sampler cannot hit every sampling point precisely and sometimes is of by +/-1. In any other resampling mode than Nearest Neighbour, this is not a problem because they look at the fractional part of the sample position as well, but it's in the definition of Nearest Neighbour that we do not look at fractional sample positions.

Hence, there is nothing we can do here. Resampling by almost three octaves using Nearest Neighbour is a very bad idea anyway. You will have to fix your sample yourself and live with that fact.

I did not intend to resample. I intended to replace 2 period by 14 period.

Please show me debug log

No matter what you wanted to do, you can manually fix the result by using the sample draw feature: Click the pen icon and correct the one offending sample point.

I don't see where the rounding error should be, and I don't see division by 7 either.
Resampling 2 to 14 samples - 14/2=7 factor
Sample 0*7=0
Sample 1*7=7
So won't that be 0 to 6 and 7 to 13 without rounding errors? I don't understand it.

I don't see where the rounding error should be, and I don't see division by 7 either.

Upsampling by a factor of 7 means that the step increment is 1/7. So OpenMPT needs to step in increments of 0.142857142857142857... through the sample per sample, and that is a periodic number as said.
Computers cannot represent periodic numbers with infinite detail, so the period has to be cut off at some point. Let's assume 1/7 is represented as 0.142857142857142857, which is already a higher precision than what OpenMPT can represent internally.
At step 0, the sample position is 0.
At step 1, the sample position is 0.142857142857142857.
...
At step 7, then sample position is 0.999999999999999999 - NOT 1 as you would expect.
You see that even at a higher precision than what OpenMPT internally uses, the sample position will still not reach the first sampling point even though if the computer had infinite precision, it would have done so. These rounding errors become more obvious over time, leading to the problem you discovered.

Piotr, this is the last warning. I told you before that your behaviour of discussing with developers about things they have already justified is not welcome. If you do this once more, I will have to remove your access to the bug tracker. If you have specific questions about resampling, ask them on the forum, but not on the bug tracker.

How about using different division, like 7/7, not multiplying 1/7 by 7?

Also, once again, I'm not registered to forum!

You can register at https://forum.openmpt.org/index.php?action=register
Or just stop commenting on this issue. I already explained everything that's to be explained.