I do not file a bug report proper because I dare not try to reproduce the bug, but I will report it here anecdotally.
I exported tags for 1084 files to mp3tag.txt. I used the default txt-file profile, but have adapted the fields to export and custom field separators. I have used this template a zillion times without problem, also on similarly large collections. Basically I use this template when I for some reason need to transcode between lossless formats but keep the tags or when other tag mysterious problems arise that require cleaning all tags and then writing them back tot he files.
So the process is a mere exporting the tags - removing all tags - reapplying the tags from the exported file.
But this time the exported files was all hodgepodge. After like 300 files entries started to switch position in the list, i.e., tags for files 325-330 were exported to position 304-309, so when reapplying them they were applied to the wrong files and the more further down the list the more chaos reigned.
One peculiar aspect of order in the chaos was that albums wre kept intact. Files from one album would not be scattered all over or separated from each other but they would be moved as a group to a new location between two other albums. Whether in fact this is due to album tags or position on the harddrive I cannot tell because albums and harddrive folders co-incide.
Thank god for the undo command which restored all tags.
But I want to point you attention to this potential problem when exporting tags.
Let me know if you need more info.