magento2.3.4 – page cache is invalidated after the “Category Products” index is reindexed

i have problem when the cronjob run, the page cache is invalidated after the “Category Products” index is reindexed.

  • this problem happens exactly after each 30min (time of cron job) the FPC become invalidated
  • -there is no changes or updates to products, categories, catalog price rules, static blocks, 3rd party extensions, etc.
  • there is no visitor to website as its under construction
  • there is more than 100K product on website

We use:

  • magento 2.3.4
  • PHP version 7.2
  • mysql Ver 14.14 Distrib 5.7.33, for Linux (x86_64)
  • VPS Cloud hosting
  • the cpanel cron job -> https://ibb.co/Wg9v2b1

any idea/solution?
Regards
Gabriel