Caption is displaying as code in page

Hello everyone,

Since updating my WordPress to the last version, I’m having an issue with captions on my site. The captions for my images are showing in code instead of using the style defined in the theme.

It is on a preproduction environment so unfortunately I cannot share the link to the problem. Here is a screenshot of what it looks like: https://i.imgsafe.org/b94d88d782.png

In the admin panel, every thing looks fine – no changes from before the update.

I have no errors on the site, no 404 or anything like it. My theme is a custom theme, I am pretty sure the problem is within it since putting Twelve eleven instead fixed it. Any ideas on where to look, how to debug?

Thanks, best community ever!