Tab/Shift+Tab are actually default shortcuts and while they do conflict with shift-selections, I cannot reproduce the error your descibe - the selection keeps disappearing after Shift is released.

I've tested the same thing on Windows 8.1 x64 today, and the problem persists for me. In attached video (available only for 24 hours) i've performed the channel switching twice, first attempt shows correct behavior, with my current mapping (TAB and CTRL+TAB for switching the channels forth and back, respectively).

Next, i've changed one of the bindings from CTRL+TAB to SHIFT+TAB, and tried to cycle between channels again, which resulted in the selection locking up again (can be clearly seen from 0000006:0000001:05 mark, when done while the track is playing, and view scrolls).

Interesting thing you might have noted, is that the dialog box, saying that the current combo is already mapped to some other action keeps popping up everytime when i'm changing this setting back to the SHIFT+TAB, even though i already confirmed removing the redundant mapping before recording the video.

I'm starting to have a sneaking suspicion that it's a conflict in my custom config, sitting somewhere, which i haven't managed to find yet (the keyboard mapping used on both OSes is the same one, imported from the exact same file)...

Video in question: https://a.uguu.se/B0p1J2n5I763_MPT_bug.mkv

Added the keymap file for reference

Observation: Only happens if Shift+Tab was previously not assigned to this shortcut, and only happens until OpenMPT is closed.