What are the sample rates of both OpenMPT and Dosbox.conf?
Is interpolation turned on in OpenMPT?

There are multiple issues with this bug report:

1. Nondescript title. Imagine if every issues was called "Houston, we have a problem" and try navigating between them.
2. Yes, you should not have posted a video, instead (or in addition) you should have posted an example module. It is impossible to recreate and verify anything without knowing the exact specifics of your example file.
3. "Something feels off" is very nondescript as well. Even if you cannot pinpoint exactly what is wrong, at least try to give an indication which part of the envelopes feel wrong by giving timestamps in the video.
4. Do not change the priority field when submitting a report. Just because you think something is important to you doesn't mean that we must fix it immediately.

Please consider these points (and also how to write a good bug report) when submitting future reports. Points 2 and 3 should be rectified already in this report, otherwise there is no way we can help.

My best guess without having any reference module to compare is that you are running Impulse Tracker with a low-quality 8-bit sound driver, maybe SoundBlaster Pro. This might give the impression of silence where there is in fact something to be heard. Compare OpenMPT output against higher-quality, interpolated Impulse Tracker output (e.g. SB16 driver with 32-bit interpolated setting chosen on Shift+F5 screen), and the output will probably sound much closer.

I believe it may happen in FT2 as well. On one tick, the envelope vol raises up smoothly that what I expected on Impulse Tracker/Fasttracker 2 in Dosbox.

https://drive.google.com/file/d/1SujJlbz-CsOTzzGTCY0wH7W9TAhPb8Zw/view?usp=sharing
https://drive.google.com/file/d/1itrF56i5LqRMWN2Y8oLe6d-2nlvIJ1eb/view?usp=sharing

Here are the 2 files, Compare them with impulse tracker and fasttracter 2 on dosbox

FYI Appels, Sample rate for all sound devices is 22050 in Dosbox. OpenMPT's sample rate is 48000 Hz.

than* (SORRY, trying to correct myself with asterisk)

And what is your sound driver on dosbox?

FYI Appels, Sample rate for all sound devices is 22050 in Dosbox. OpenMPT's sample rate is 48000 Hz.

So you are comparing apples (sorry) and oranges.

On one tick, the envelope vol raises up smoothly that what I expected on Impulse Tracker/Fasttracker 2 in Dosbox.

There is absolutely no standard regarding volume ramping. It's not even consistent across audio drivers in Impulse Tracker, and while the envelopes are a lot more clicky with IT's SoundBlaster drivers, its WAV writer is a lot more comparable to OpenMPT's output for example. The GUS driver is also not quite as clicky. The ITSB16.MMX driver is also less clicky because it supports volume ramping like OpenMPT does. OpenMPT strives to offer the best quality, not to emulate low-quality SoundBlaster drivers.

If you feel that OpenMPT's ramping is still too soft for your taste, you can lower it. But there is no bug here. You will find that XMPlay sounds very comparable to OpenMPT as well.