Outer Glow transparency lost when converted to PDF

Hi there,

I'm trying to place white text over a photo. To make it show up better, I've applied a dark colored outer glow set to be transparent (multiply). When I export it as a PDF, the glow shows up opaque. This is not apparent when viewing in Acrobat, but shows up in file previews, and arrived at the print business in the same condition. I used the same technique throughout the document, but the problem isn't happening on every page.

I've tried various combinations of solutions (changing the transparency blend space, transparency flattener, saving as an interactive PDF, changing CMYK process colors to spot colors, changing to RGB colors). I've seen suggestions that one should choose High Quality Print > Advanced > and select High Resolution in the Transparency Flattener - but it's grayed out for me. Nothing that I've tried has worked.

I have used this outer glow technique for years without issue, but it cropped up a few months ago when I was working on a similar assignment. I don't know what changed.

I'm using InDesign 17.1 on a Mac, Big Sur 11.4. I know there are compatibility issues between Adobe and the new Mac M1 processors, but this problem did not immediately pop up when I started using this computer - so maybe not the issue?

I've included two screenshots. The first one shows how it appears within Adobe InDesign and Acrobat (the way I want it, with dark transparency around the text), the second how it appears outside Adobe (with the opaque glow that I don't want).

I'm not a professional graphic designer but have used InDesign for many years - meaning that I don't know the finer details of InDesign, but have not had problems before. Any help would be appreciated. Thank you!

Amelia