Converting from 48 kHz to 44 kHz looses the highest frequencies [with various software]

I had this problem ever since, but it did not bother me in the past as I did not knew what exactly it meant. It goes like this

[Below I am referring to my latest case of this problem - if particular files for testing will be needed I can provide them; but I think this canbe discussed without even seeing the data]

I use DVD Audio Extractor 7.1.1 [and before that version 6.3.0] to extract stereo PCM WAV files. I set them to be saved as 48 kHz. As a result the files I get have all the frequencies visible on the spectrogram, going almost to the very top [~23.5 kHz]. But if I extract the same tracksto a 44 kHz WAV file, then the frequencies in them will go only up to ~20.5 kHz. So this is a problem of DVD Audio Extractor? I wish, it was so

Because I tried converting those better ones WAVs [48kHz] into FLACs and WVs, with both NCH Switch Sound File Converter Plus 4.35 and AIMP Converter 4.51 2080. The result was more or less the same, with minimal variations- i.e. a profound lost of high frequency data. I also tried converting those 48 kHz WAVs in Audacity 2.3.2 and Sony Sound Forge 7.0 to 44 kHz- as a result the new WAVs also lost the frequencies above the 20 kHz. And all those losses do not look like a ordinary plain cut-off line- instead of that they look "natural", as if the audio was recorder / released in that way [as opposed to being crassly compressed somewhere further down the line]

So how can this be happening? I've seen this through years over and over with all kinds of files from various sources: spectrogram shows "everything" when the file is 48 kHz, but after conversion / re-sampling it becomes more inferior than it should [loosing not only that what was above the 22 kHz level]. And yes: I double checked the settings of the output formats - it is not a case of wrong settings in multiple pieces of software; to which attests also the fact that I do not have no such problems when converting 44 kHz files that have full frequencies [they retain frequencies up to 22 kHz]

Read up on the Nyquist Limit.