EoIP and GRE protocol

hello!

i have tried to search old post but cannot find any solution.

here we have 2 MT RouterOS, one at office, another at home

MT RouterOS A at office:
lan 192.168.0.1/24
public 204.1.y.z

MT ROuterOS B at home:
lan 192.168.10.1/24
public 204.2.y.z

i have the following rule in INPUT chain for protect purpose (reference as MT demo)
chain=input connection-state=invalid action=drop
and i also have accept GRE protocol in both INPUT and OUTPUT chain

but EoIP fail if the “connection-state=invalid action=drop” rule is before than “accept GRE protocol” rule. if i put “accept GRE protocol” rule before then EoIP success.

how come GRE protocol meet “connection-state=invalid” ? could anyone explain? thx!