It doesn't matter if I play Practice or Veteran, each time "Cut a Pipeline" mission starts with only 1 of mags, grenades, rockets, bombs and aid kit. What is even more annoying, collecting US Armby supply bags doesn't give you any supply so I have to rely on what I started mission with.
Anyone knows how to fix it?

Never heard of that bug. That sucks.
I assume, playing on "easy" (facing less enemies) doesn't really help? Other than starting the mission anew and hoping for it to be fixed automatically? No.

Maybe use an older save game, replay one or two missions prior to the "Cut a Pipeline" mission (and again: hope the bug won't occur this time)?

Edit: typo

Nah, I think it's deep in the game.
I have reinstallad the game, try it as practice ranger, then veteran. I even tested one of the "online free versions" - the same issue.
Wonder if Steam version has the same problem.

Is the game cracked? I mean, does it bypass the protection codes?

I ask this because Atari already messed up with Gunship (another Microprose game, from 1986) release with a cracked copy which made the game malfunction.
They later patched up that game with a clean copy.

Maybe Airborne Ranger has a similar problem, a crack that messed up some game variables.
It that's the case, you'd have to find a clean, unadultered copy of the game to test it.

Edit: Ok, forget what I said. The code protection is active. Have you tried the campaign mode to see if the problem also happens there (unfortunately someone would have to beat a series of missions before reaching 'cut the pipeline') ?

The game seems to be bugged according to this old FAQ from 2005:

https://www.ign.com/articles/2005/01/31/airborne-ranger-general-faq-583853

Actually, the writer says there was a cracked version which fixed this bug. Oh well...

Edit:
Ok, I've been digging some more, and I see the Atari/GOG version is 441.01.
According to some [cough, cough] DOS preservation projects, there is a 441.03 version. But there's no guarantee they fixed this bug.
I don't know how to find this version, but I'll leave this information here as it may be useful for someone.

To be fair, I'm really tired of these low-effort re-releases for a quick cash grab.
I mean, c'mon, you don't even test your product or ~worse~ : don't even care to find the most up to date version.

Some months ago I had to find a pirate copy of Echelon (Access Sotware, 1988), because the version I paid for was outdated and had a CRITICAL BUG right before the ending sequence.
I downloaded an updated version (oficially released back in the 80s) and ta-da, it was really fixed and I finally finished the game.

Ok, I don't know how to fix it, but I found a workaround:

The problem lies within the combination "copy protection screen + cut a pipeline mission".

So, if you need to play this exactly mission in, let's say, the campaign mode, FIRST you need to start another mission to enter the protection code (Riibbon screen). I did this: enter Practice Mode, choose any mission except "cut a pipeline", put the protection code, kill myself jumping out of the screen from the airplane an so I can return to main menu. THEN I return to the campaign mode with my saved Ranger.

This make sense. I just played this game casually, one mission at the time.
Thanks a lot, I will try this out!