Random 404 errors on category pages (while using a cache warmer)

[TimQSO]

Magento 2.3.3
Amasty Improved Layered Navigation
PHP 7.3
Nginx 1.17

Our cache warmer registers random 404 errors while warming pages at higher speeds. 404 errors are mainly registered while trying to warm up category pages.

The default Varnish VCL generated by Magento also caches 404 pages. Resulting in inaccessible categories that return a 404 to the customer for a long period of time. Excluding 404 pages to be cached by Varnish will limit the effects of this issue, because on a retry the category could return a valid 200 status.

Setting "Remove website code from store url" to 'No' solves the issue, no 404 errors anymore.

Our websites rely on Amasty Improved Layered Navigation and I suspect that your module interferes with it. However, I was not able to find the cause.