Affinity Publisher: filled shape across spread triggering bleed hazard alert

[frindley]

I'm working on a 24-page A4-size book that will be saddle-stapled. (So no bleed required by printer for binding on inner edges of pages.)

On one spread I have created a full-spread rectangle (allowing extra 3mm for bleed on all outer edges, so A3+ size) and filled this to achieve a coloured background. 

On another spread I have a full-spread but half height rectangle (so about 18mm H by 426m W), again filled to create a coloured background for reversed text.

Both of these appear to be triggering a "bleed hazard" alert in Preflight (anchored in each case to the right-hand page of the spread). There's no text anywhere near the relevant edges. 

A similar treatment on the centrefold spread (A3+ size photograph occupying the full spread) is not triggering this alert, presumably because it will be printed on a single sheet. 

How do I solve the bleed hazard alert?
Do I need to adopt a different strategy to achieve the coloured background running full width of the spread? 

Edited March 18, 2021 by frindley

[Joachim_L]

This warning / error just means that an element is near the bleed zone. You can a) ignore the warning / error if you think everything is correct or b) deactivate the bleed hazard check in the Preflight options or c) change the settings for bleed hazard in the Preflight options.