Change to p2p on both sides, on SRX
set protocols ospf area 1.1.1.1 interface gr-0/0/0.0 interface-type p2p
Change traceoptions to flag "all" run the test and search trace file for MTU. You should see the exact values used by SRX and Mikrotik. If there is a mismatch please correct accordingly
Regards, Wojtek