I reverted back to lp:mixxx r3128 and the issue does not appear.

The following was tested with mixxx-1.11.0-alpha2-daschuer~daschuers-trunk-bzr2979-macintel32.dmg

#1 = start simultaneous playback - press "D" & "L"
Result: Only the deck triggered by "D" plays

Debug output Mac OS:
Debug [Main]: KeyPress event = 76 KeyId = 1 
Debug [Main]: keyboard press:  "L" 
Debug [Main]: KeyPress event = 68 KeyId = 1 
Debug [Main]: KeyRelease event = 76 AutoRepeat = false KeyId = 1 
Debug [Main]: KeyRelease event = 68 AutoRepeat = false KeyId = 1 
Debug [Main]: KeyRelease event = 68 AutoRepeat = false KeyId = 1 
Debug [Main]: Committing transaction on "qt_sql_default_connection" result: true 
Debug [Main]: PlaylistTableModel(0x614a5a0) select() took 1 ms 

#2 start simultaneous playback - hold Hotcue "x" & ","
Result: Only the hotcue triggered by "X" plays and stops once "," is released

Debug output Mac OS:
Debug [Main]: KeyPress event = 88 KeyId = 1 
Debug [Main]: keyboard press:  "X" 
Debug [Main]: KeyPress event = 44 KeyId = 1 
Debug [Main]: KeyPress event = 44 KeyId = 1 
Debug [Main]: KeyPress event = 44 KeyId = 1 
Debug [Main]: KeyPress event = 44 KeyId = 1 
Debug [Main]: KeyPress event = 44 KeyId = 1 
Debug [Main]: KeyPress event = 44 KeyId = 1 
Debug [Main]: KeyPress event = 44 KeyId = 1 
Debug [Main]: KeyPress event = 44 KeyId = 1 
Debug [Main]: KeyPress event = 44 KeyId = 1 
Debug [Main]: KeyPress event = 44 KeyId = 1 
Debug [Main]: KeyPress event = 44 KeyId = 1 
Debug [Main]: Committing transaction on "qt_sql_default_connection" result: true 
Debug [Main]: PlaylistTableModel(0x614a5a0) select() took 1 ms 
Debug [Main]: KeyPress event = 44 KeyId = 1 
Debug [Main]: KeyPress event = 44 KeyId = 1 
Debug [Main]: KeyPress event = 44 KeyId = 1 
Debug [Main]: KeyPress event = 44 KeyId = 1 
Debug [Main]: KeyPress event = 44 KeyId = 1 
Debug [Main]: KeyPress event = 44 KeyId = 1 
Debug [Main]: KeyPress event = 44 KeyId = 1 
Debug [Main]: KeyPress event = 44 KeyId = 1 
Debug [Main]: KeyRelease event = 44 AutoRepeat = false KeyId = 1 
Debug [Main]: KeyRelease event = 88 AutoRepeat = false KeyId = 1

#3 start playback - hold Hotcue "x" and later press or hold ","
Result: Hotcue triggered by "X" plays and stops once "," is released even if "X" is still pressed

Debug output Mac OS:
Debug [Main]: KeyPress event = 88 KeyId = 1 
Debug [Main]: keyboard press:  "X" 
Debug [Main]: Committing transaction on "qt_sql_default_connection" result: true 
Debug [Main]: PlaylistTableModel(0x614a5a0) select() took 1 ms 
Debug [Main]: KeyPress event = 88 KeyId = 1 
Debug [Main]: KeyPress event = 88 KeyId = 1 
Debug [Main]: KeyPress event = 88 KeyId = 1 
Debug [Main]: KeyPress event = 88 KeyId = 1 
Debug [Main]: KeyPress event = 88 KeyId = 1 
Debug [Main]: KeyPress event = 88 KeyId = 1 
Debug [Main]: KeyPress event = 88 KeyId = 1 
Debug [Main]: KeyPress event = 88 KeyId = 1 
Debug [Main]: KeyPress event = 88 KeyId = 1 
Debug [Main]: KeyPress event = 88 KeyId = 1 
Debug [Main]: KeyPress event = 88 KeyId = 1 
Debug [Main]: KeyPress event = 44 KeyId = 1 
Debug [Main]: KeyPress event = 44 KeyId = 1 
Debug [Main]: KeyPress event = 44 KeyId = 1 
Debug [Main]: KeyPress event = 44 KeyId = 1 
Debug [Main]: KeyPress event = 44 KeyId = 1 
Debug [Main]: KeyPress event = 44 KeyId = 1 
Debug [Main]: KeyPress event = 44 KeyId = 1 
Debug [Main]: KeyPress event = 44 KeyId = 1 
Debug [Main]: KeyPress event = 44 KeyId = 1 
Debug [Main]: KeyRelease event = 44 AutoRepeat = false KeyId = 1 
Debug [Main]: KeyRelease event = 88 AutoRepeat = false KeyId = 1