Why is one image trying to load over http and not https?

Question

Drupal

Hello. There is a strange bug
Website on drupal 7.61,
php 7.2,
Apache web server
ssl certificate from let's encrypt
in .htaccess

RewriteCond %{HTTPS} on
  RewriteRule ^ - [E=protossl:s]

in apache.conf

RewriteEngine on
                RewriteCond %{HTTPS} off
                RewriteRule (.*) https://%{HTTP_HOST}%{REQUEST_URI} [R]

And there is Strict-Transport-Security

Header set Strict-Transport-Security "max-age=31536000; includeSubDomains; preload"

What is the problem?
Sometimes, after clearing the cache, one resource from the main page, namely the Lazyloader Placeholder Image.gif, tries to load on an http request. What breaks the site and after failure, some images are not loaded.
In the module settings, I tried to manually specify the path to the placeholder image in the Placeholder Image https field. - Unsuccessfully. Is not downloading.
Looked at the code of the module. There is no hint of https or https.
I don't understand what the problem is, why one single module image tries to load via http and breaks the load, making mixed content.
Thank you.