Incorrect Transparency Blending Space

Hello. I need to export my document to the internet in the correct color mode RGB.

I have all my colors in the swatches panel in RGB, as well as my images, and the Transparency Blending Space also in RGB, even in Document Settings I have set "Web". But with everything correct, the Preflight panel tells me "The transparency blending space is incorrect." So, having everything in RGB, if I set the "Transparency Blending Space" to CMYK, the error disappears.
Why is this happening?
I have other documents that are not so clean in color, that do not show that error. What else should I pay attention to for to not have error?