Nobody is going to be able to help you as long as you are using those ridiculous fake domains. If you need to hide a domain for privacy reasons, please use one of the reserved names, example.com, example.net, example.org so that it is obvious. Note that hiding the real domain name can often make it impossible for anyone to assist.
If you haven’t already reviewed your server logs, that is going to be the best place to start.
I am extensively using subdomain feature on HestiaCP since many years. I have more than a hundred subdomains for testing purposes. These domains have very minor variations, like xyz1.domain.com, xyz2.domain.com, etc. I never ever had any problems.