Hi Dmitri ,
Thank you once again for this great software and amazing chance to help make it fabulous.
I ran an extensive test yesterday process of Music Library >> Tools >> Process Track
The test library consisted of around 26K files I ran the 'Set BMP value in tags' without the boxes ticked as below

This finished leaving the -1 on screen in the Music library windows and seems to have not changed the records.
I selected these records and tight Click Menu "Exported to Player " This strangely exported all the tracks not just the selected . I could not sort on BMP to see how many lines I had so printed the playlist adding %bpm to the template I dragged the the play list to Excel as HTML and could then sort on BPM this gave me 552 titles with -1 .
Then i checked no value these items in the playlist which ranged from cells 2946 to 26910. That is 23964 MP3 files which had not been changed or the information removed and a blank value substituted It is fair that some of these cannot be set as some are NOT MP3 format but is not the figure 23364 which I am being presented with .
I hope this helps as something seems wrong to me