You could be right. I get the above results with the .st image of Pompey Pirates #22 in Steem 3.2 using UK TOS1.02 in each case.
It's always possible the Pompey code (or even the emulator) has some effect on what happens, and for all I know Mastertronic may have fixed it for 1Meg. I still have the original disks, so if they're still readable I might get around to testing it on the STFM. The machine started out with 512K, but I'm no longer certain what it was upgraded to. At the time I thought the problem was to do with mouse/joystick combination. Possibly the joystick fire button should also advance the menu, but this is just vague recollection.
I discovered Steem (and the availability on the web of all the old ST favourites) only a couple of weeks ago, and before realising you could fix Fighter Bomber by changing the RAM size, I was busily disassembling the Pompey loader (with Steem debug build), up to where it loads (and presumably decompresses) the Mastertronic code. Further delving in that direction might clear up exactly what the problem is, but I'm not promising to do it.