When I export a structure, some blocks don't get saved, most notably those blocks being stairs/slabs.
For some reason, some of the slabs will export, but majority of them won't.

the blocks that are excluded also seems to be deterministic and apply from the upper south edge toward the opposite edge. the only way I've managed to work around this is either by making very small structures or if a structure has to be large, starting out with a very limited block palette. this bug basically cripples any ability to make large detailed structures, even though preexisting ones are just fine.

(it doesn't happen to every structure, but I can't figure out what causes it and it happens often enough that it's fatal.)

nope - this happens with vanilla blocks (shown with quartz slab in the OP and tested with various others myself) and with no transformers active. it appears to become more common with the size and complexity of the structure, and vary based on whether it generates rotated or not. the blocks affected seems to be based on what type of placement logic they have; ones with basic or log-like placement logic are fine, while ones that morph or rotate in more complex ways are not, for example stairs, panes, walls, fences, and slabs are affected but full blocks and plantlike blocks are not.

scratch that, actually - it seems the bug only occurs when the structure is rotated, even happening when using the /#paste command. it can be worked around by making 4 variants of the same structure facing in 4 different directions, setting them all to non rotating, and giving them 1/4th weight. this gives the illusion of one normally weighted rotatable structure.

This can only happen if the mod in question is doing weird stuff. All blocks are exported and handled by ReC by default. Slabs, stairs etc. are handled especially and used in many of the default structures.

Either that or you're using (unknowingly) a ruins transformer in your structure.