I have some products with "Layout Update XML" Design updates (simple XML pointing to a template file; very basic and in the correct format). Every 24 hours after a cache flush, the code vanishes from the frontend website. The code still exists in the backend product's "Layout Update XML" section, but the frontend website does not show it anymore. Why is this?
I am using Magento 2.2.5 with Varnish to the cache. The Varnish (as per its default setting) seems to clear its cache every 86400 seconds (24 hours). It seems that when it clears itself, the new versions of the website it shows are without the code in the Layout Update XML.
If I run CLI commands "cache:flush" & "cache:clean" and then "service varnish restart", my custom Layout Update XML code begins to show again. However, 24 hours later, it disappears again from the frontend.
Note: this issue only occurs in "Production Mode". In Development Mode, the XML block update never disappears.
What am I doing wrong here? Do you have any advice on how to correct this or, at least, where to look?