Where does Mikrotik get it’s masquerade “from” address?
Does it come from the default route IP?
Does it come from the outgoing interface IP? If so what if I have multiple IP addresses on my outgoing interface, can I tell the mikrotik to use only one of those?
If the out interface have more than one IP, the mangle choice the one with less hexadecimal value, like bridge on various interface without force admin MAC, it choice the MAC with less value.
For example (exagerated):
If you have 1.2.1.4 and 66.55.44.33, obviously choose 1.2.1.4,
and if you have 4.1.1.2 and 3.175.248.250 choose 3.175.248.250.
For force the out with specific IP address you must set the IP if you want use on Pref. Source on Route.