My question is, should external requests arrive to an internal LAN address as if they were coming from the router.
example:
internal address: 192.168.1.10 port 9090
router ip: 192.168.1.250
if I request this port from the outside, it arrives as if it came from the router.
Thanks! 
It can happen if you configured your router to do that. If you don’t want it to happen, then you misconfigured it.
You probably have unconditional masquerade rule, but normally you want one with out-interface=.
my question is exactly
How to “regenerate” traffic so port forwarding to some internal LAN host is not visible as connection coming from the outside network, but from the router itself?
Well, if you do want to see router as source, then add srcnat/masquerade rule that covers this traffic, e.g.:
/ip firewall nat
add chain=srcnat out-interface=<LAN> action=masquerade
Depending on how complex network you have, it may be ok, or it may need some fine tuning.
You are sufficiently obtuse that unlike SOB I will not guess nor will I attempt an answer without a better explanation.
This occurs when one tries to communicate a requirement by a config option… , dont mention the config and only use cases.
Either use some examples with or without a network diagram.