Mmh. Maybe something with bundles, params, safaHTML settings, config, links, resources…
If I were you I would extract just one example thats runnable and works with 115. Someona can replay and compare with 135…
Without too much guessing. And I’m pretty sure in general iframes are working.
maybe this one for Hugo 123 is related Companion page-bundle HTML files not being copied to subfolder of output directory anymore