Yes, you should not run either EX2200 or EX3200 on any 15.1 code. Therefore I still suggest you try 12.3R12-S7 or S-latest.
This lPR ooks to potentially be an issue introduced in 15.1 code stream, as discovered against 15.1R2, but supposedly/maybe not seen in 15.1R1.
You should NEVER run an F release on any EX Platforms. F releases are specific to MX/PTX (I believe), and add no additional capabilties to EX platforms.
I still believe for all legacy EX platforms, anything that is non-ELS, you best bet is to stick with some recent flavor of 12.3 at this time.