I'd like to add some more info. The Concordance Search in Trados Studio 2021 seems unable to search for specific words. Could you please try and search for words like "customization" or "personalization" using a TM that for sure contains these words please? In my case, when I approved a segment with those words, the Concordance finds them only in this segment. But when the same TM is used in version 2019, it provides many more hits. Thanks for checking!

Hi Grzegorz Stasiak

While there could be many factors please let me know if you see error messages in and around the TM results pane, such as:
There was a problem while searching. See the Messages Window for more information. > An error has occurred while using the translation provide "TM Name"

Have you tried to re index/upgrade your TM?

Lyds

Hi Lyds. No there are no warnings or errors. It apparently works fine. And I would not notice it if I wasn't sure that the terms in question were in this specific TM. I tried to export the original TM from Studio 2019 using different options to no avail. For the time being I stopped using Studio 2021 because I don't remember all words in that TM it doesn't show which may be dangerous for consistency.

Grzegorz Stasiak - when you remember a few good examples and if you are willing to share your TM please do so I can take a close look at it.
In the meantime, please remember there are search settings would could explain why 1 version seems more accurate over another 

Hello! After some time I decided to check this issue again. After some testing I found out that this problem disappears when I create a new TM in Studio 2021, select "Enable character-based concordance search" and import any older TM from previous Trados versions I need. So far I've tested it only on the TM that created problems but it looks promising. The strange thing is that the same TM in Studio 2019 has this option unchecked and works fine.

One more interesting thing I noticed. When I initially spotted this concordance search issue, I was looking for the word "customization" which I was sure had already been translated and should have been present in TM but Studio 2021 could not find it. I remember that the project source language was set as en-GB and the imported/connected TM also was en-GB. So I exported the TM, changed this word to "customisation" in a Notepad and imported the TM again to the same project. And guess what? Suddenly the concordance could show it.

I don't know if this is a bug in Studio 2021 and if it refers just this one word but I must say this is the strangest thing in my over 22 years long experience with Trados.

Grzegorz Stasiak thank you for your update 

I have finally got round to installing Studio 2021 (I delayed because I heard of bugs), and am already regretting it as I have encountered this very same problem! It cannot find terms in my TMs that I know very well are there. Has a fix been found for this in the intervening 3 years? Or am I really going to have to created a new version of each of my TMs and then import the old version?
I have my fingers crossed that there's a faster solution.

I have finally got round to installing Studio 2021 (I delayed because I heard of bugs), and am already regretting it as I have encountered this very same problem! It cannot find terms in my TMs that I know very well are there. Has a fix been found for this in the intervening 3 years? Or am I really going to have to created a new version of each of my TMs and then import the old version?
I have my fingers crossed that there's a faster solution.