Selecting multiple tracks problem MAC

Hello all, I use Mixxx 2.3 on an iMac, Big Sur 11.5.2, 32GB Ram, Intel i7
When selecting multiple tracks from a playlist or Auto DJ to move to another playlist/delete etc., the selections jump about, by which I mean items I have selected are deselected and items I haven’t are selected; for example, say I select every other track for six tracks, I select a seventh track and all the tracks I have selected are unselected and 3 or 4 above the seventh are selected in a block. I have replicated this on another iMac, so I know it is not the mouse or keyboard.
This does not occur when selecting the files when in the main Tracks screen.
I am wondering if there is a setting somewhere that needs changing or something else. Thanks in advance for any help offered.