From results with IPSEC/GRE we can see acx1100 have some limitations related to ipsec, a reason can be the missing services ipsec-vpn rule RULE_NAME match-direction output . If we remove ipsec and do the configuration only with gre we will see no issues between mikrotik(PC1,PC2) and acx1100(LR1,LR2,LR3) routing.
Still we can say acx1100 ipsec is usable for tasks like ipsec/esp encrypted management access to remote acx1100 and probably a lot of other scenarios.
Probably I miss something and the configuration in question can work, however Juniper ACX main function is to do routing and not to be used as security gateway.
If ipsec is needed, Juniper SRX can be a good ACX companion 🙂