I use 'beets' to keep my tags in order.
In the current changelog I find this entry from 1.3.2 (December 22, 2013):
When writing ID3 tags for ReplayGain normalization, tags are written with both upper-case and lower-case TXXX frame descriptions. Previous versions of beets used only the upper-case style, which seems to be more standard, but some players (namely, Quod Libet and foobar2000) seem to only use lower-case names.
That was a while ago. But maybe I now have tags in upper-case and lower-case, while Poweramp only accepts lower-case.
Any idea what tool I can use to view the tags byte by byte in the original? Hex editor?
Tom