Hi @sd5e8a , When you add any transparency effect or transparent object to a page the document’s Transparency Blend Space is invoked for the entire page. At output a page with live transparency has to be flattened into a single output space—InDesign lets you mix color spaces on the same page and within transparent groups, but the output can only be to a single color space.
If you prefer to handle the flattening into a uniform space at Export, you can do it with a flattened preset, e.g. PDF/X-1a.
Here I have the two out-of-gamut green fills on the same page with the doc blend space set to CMYK. If I select one and set the blend mode to Multiply, both fills are displayed in the CMYK blend space even though the one on the right has no transparency applied:
If I export the page with live transparency (PDF/X-4) and inspect the fill set to Multiply, I can see the first Transparency Group is the page, then there is the rectangle set to Multiply as another Transparency Group, and finally the RGB fill. The Blending Color Space is DeviceCMYK, so the RGB fills display in the Output Intent CMYK space:
If I inspect the other fill there‘s the page as a Transparency Group, but the RGB filled frame is not part of a Transparency Group: