I have searched my issue in different forums and couldn't find anything related to it. Hopefully someone can help me here

I work for the translation/localization company and we have clients sending us work in Flare. To prep the files for translation and import the translation, we use a combination of Flare (2018 r2) and Lingo (10.2.6786.30644). Everything goes well, I import the Flare project in Lingo, create XLF bundles, I put the whole structure as is in Trados and send to translation. My problem occurs when I need to merge the bundles back in Lingo. If I generate the Lingo XLF with Trados 2015, the merge in Lingo works, but if I generate the files with Trados 2021, the merge in Lingo doesn't work. Doesn't matter in which version of Trados I prepare the files, I tried preparing the files in Trados 2015 and 2021 and the result is the same.
I am wondering if anyone is aware of any incompatibility with Lingo and Trados 2021. Is it because the version of Lingo is not recent enough?
I use the Default template in Trados to process the Lingo XLF files, would the problem be there?
I did compare an output of Trados 2015 and Trados 2021, and tags (important ones) are not present in the Trados 2021 version - see screenshot attached. If necessary I could compare more files and generate reports.
Thank you in advance for any input!
Best,
Chris