I have the same problem. I migrated my site from a local server to a hosted server.
Before, the simulations and videos that I had embedded in each lesson worked perfectly using iframes. Now, nothing works – it just appears as a blank white section where the embedded content should be.
Embedded content works perfectly in the Posts and Pages sections of the site. But doesn’t work in the Learn Press Courses and Lessons.
I migrated my site at approximately the same time as updating WordPress to 5.4.1.
