Using trackpad gestures to enter Cmd-Delete into Spark causes not only the current message to be deleted, but also the next message. The second message can be recovered with the Undo action. Manually entering Cmd-Delete into Spark works as expected, only deleting the current message.
A similar gesture to enter Ctrl-Cmd-A to Archive the current message works as expected, only archiving the one message.
iMac Mojave 10.14.4, Spark 188.8.131.525, BTT 2.801