Oh I see what’s going on, those are sprites. I believe you can get around this issue with adjusting the translucency sorting policy. Head over to Project Settings -> Engine -> Rendering -> Translucency then change the Translucent Sort Policy to Sort Along Axis then adjust that axis to your needs. This should remove the issue altogether in your case.
Edit: Also for reference, you can have translucency/masking/transparency or 3D planes as well.