what kind of files are these?
What did you try to get tag data?
Does it work to add tag data when you enter the data manually in a field without the help of a converter?
Windows File Explorer is the worst app you can use to show metadata.
I suggest to use one of the many recommended specialized players or apps for this purpose.
If you see the metadata included in your files in Mp3tag everything is fine.
The same metadata will be shown in every decent metadata app too.
Correct. Microsoft went in late in the game to support FLAC, and their implementation remains poorly set up. Explorer is not a good method for checking tags for FLAC, as well as many other more detailed metadata fields in other formats as well.
There is. Microsoft has no financial skin in the game to support FLAC, so they have spent little time doing more than just the basics to get the files to play. Even using their player evolved from WMP has much to be desired.
I guess it depends on what your DAP expects from the metadata. If it is Android based, there are many that use common player apps. And these will typically use the metadata that mp3tag writes. I use an iBasso DAP with GoneMAD, Poweramp, Neutron, and Foobar. These all work without issue. The native Mango player uses the same tags as well but I rarely use it.