This is Match repair

звучит так интересно, многословно, и не понятно. Спасибо

What is not clear? Can you see the orange wrench icon on your screenshot? That is Match Repair. Click it and put Off for every option.
Also you can undo the repair feature by pressing Ctrl+Alt+U. Try this article: signsandsymptomsoftranslation.com/.../

RE: where it takes that IEC = МЭК
It takes it from your source text. Trados compares your new source text against the previous one and finds that the difference between them is IEC (previous source text) vs МЭК (new source text). Hence it substitutes МЭК for IEC to "fix" the target output and "minimize" your effort.

What is not clear? Can you see the orange wrench icon on your screenshot? That is Match Repair. Click it and put Off for every option.
Also you can undo the repair feature by pressing Ctrl+Alt+U. Try this article: signsandsymptomsoftranslation.com/.../

RE: where it takes that IEC = МЭК
It takes it from your source text. Trados compares your new source text against the previous one and finds that the difference between them is IEC (previous source text) vs МЭК (new source text). Hence it substitutes МЭК for IEC to "fix" the target output and "minimize" your effort.

Thanks, I will give a closer look into your link

Meanwhile, what if I need to clearly see such Match Repairs and not overlook them in pretranslation (as such segment gets confirmed), is it one of these two ticks that I need to use? :

My match repair settings window shows Batch Tasks = OFF.  Shouldn't this exclude any  match repairs in my pre-translated results above?

Alexander The Great 

I don't think this is coming from Match Repair.  If it was you would have that small orange wrench symbol next to the fuzzy score in the TM results window, and you don't.

Alexander The Great said:

Meanwhile, what if I need to clearly see such Match Repairs and not overlook them in pretranslation

You would... you would see the wrench against the score.

On the problem... it's hard to give a conclusive answer for several reasons:

1. we don't know if it's the same TM? You obscured the TM name in one screenshot, and then showed it in another.  But we don't know if they are the same TM.
2. it may be related to acronym recognition, but you also hid that part of the screenshot so we can't see if it was recognised

Given you say it's not in your TM what happens if you disable your TMs and then use a brand new TM?  Add one TU that uses IEC in the translation and the source, then see if a source with МЭК is recognised automatically.