The subject says it all.

use 8086
push EAX ; <--- this shouldn't assemble, but does on FASM2

There has to be a way to do this? It seems pointless to support a "use" directive if the assembler won't limit the available instructions.

No... I'm not going through every instruction I write making sure it's supported on the 8086, that's the assembler's job. ^_^

There's been an omission in PUSH/POP specialized handlers. Fixed already.

There might be more of such simple mistakes - I believe very few people have been using fasm 2 previews enough to find them (compared to fasm 1).

PS. I also recommend instead of plain "use 8086". While "8086" only changes the choice of base CPU, "i8086" is a collection of settings that also reset the choice of FPU and CPU extensions.