Hi, I won't go into much detail since posting whole set of rules from my router is a little overboard for this specific routing problem.
Also I am using winbox so can't provide full dumps at this time.
My original and working setup:
ether1-wan - set with DHCP-client and enabled default route. For sake of privacy, this network would be 99.99.99.0/24.
In route table, it correctly shows default route - DAS 0.0.0.0/0 reachable 99.99.99.1 ether1-wan
There are mangle rules which override routing for some specific sites or internal IPs, detailing them would take too long, I think they aren't related to my problem.

Desired setup: I want specific IP (or list) to be routed to ether1-wan regardless of existing rules or routing marks. So, as a test I add a static route to 8.8.8.8 via gateway 99.99.99.1.
Now, the problem - this static route is always inactive due to 'unreachable' gateway. Removing gateway check altogether does not help.
What is the problem here? I can successfully ping 99.99.99.1 from mikrotik itself and from internal PC.

The global idea behind creating this rule instead of using 'main' routing mark is to ensure that specific site(s) would be fetched through this exact interface, regardless of any future changes of default route. Like, at some point I'll want to route all traffic through VPN and still this site should be forcibly routed to ethernet interface.

//edit.
Problem solved, probably I messed up at creation of route. Deleting and adding it again fixed the thing