Sorry, I’ve mixed up several topics.
Sorry again (same reason), in your case that rule is redundant as everything is always src-nated thanks to the rule
action=masquerade chain=srcnat ..no other conditions…
Leaving aside whether such wide-matching rule is a good idea or not, the reason of your trouble must be different. In fact it’s the policy routing again - you assign the routing-mark PureVPN-PPTP to everything from 192.168.12.0/24 via the VPN “WAN”, but there is no specific route to 192.168.12.1 (or 192.168.12.0/24) with that routing-mark, so the marked packets take the default route with that routing-mark instead. So the fastest way out is
/ip route rule add dst-address=192.168.12.0/24 action=lookup-only-in-table table=main
More details here.