I had the same problem. Found this:
If you apply transparency to objects on a spread, all colors on that spread convert to the transparency blend space you’ve chosen (Edit > Transparency Blend Space), either Document RGB or Document CMYK, even if they’re not involved with transparency. Converting all the colors results in consistency across any two same-colored objects on a spread, and avoids more dramatic color behavior at the edges of transparency. Colors are converted “on the fly” as you draw objects. Colors in placed graphics that interact with transparency are also converted to the blend space. This affects how the colors appear on‑screen and in print, but not how the colors are defined in the document.
Depending on your workflow, do one of the following:
- If you create documents for print only, choose Document CMYK for the blend space.
- If you create documents for web only, choose Document RGB.
- If you create documents for both print and web, decide which is more important, and then choose the blend space that matches the final output.
- If you create a high-resolution print piece that you’ll also publish as a high-profile PDF document on a website, you may need to switch the blending space back and forth before final output. In this case, be sure to reproof the color on every spread that has transparency, and avoid using the Difference and Exclusion blend modes—these modes can change the appearance dramatically.