maybe it's an anomaly to do with the query itself being in a tabbed form.
can you send a picture of the entire screen.
I have lots of different dummy bsv's that I use for testing and bug finding.
I can probably simulate your layout quickly and confirm if it does it for me, and narrow down the conditions under which it occurs.
also, I suggest doing an experiment:
Add a Delete PANEL operation button to the query at the top , but tick the "Applies to Multiple Items" checkbox in the dialog for adding the operation, so that a column of checkboxes is added which can be ticked to delete multiple items.
Does it still do it then?