Bug: `alternative_sprites` in conditional blocks away from the parent behave inconsistently

As alternative_sprite blocks can be placed away from their 'parent' sprites, the NML syntax implies you can do things like define alternative_sprites in a conditional block which are used only when that condition is met. For example:

replace grid_temperate_spr3981(3981, "temperate_groundtiles_gridline_8bpp.png") { template_groundtiles(0, 0, 1) }
if (climate==CLIMATE_ARCTIC) {
    alternative_sprites (grid_temperate_spr3981, ZOOM_LEVEL_NORMAL, BIT_DEPTH_32BPP, "arctic_groundtiles_gridline_bt32bpp.png") { template_groundtiles(0, 0, 1) }
}

This doesn't work as you'd expect from the code. The alternative sprites are always used irrespective of climate.

The syntax also allows some quite nonsensical definitions:

if (climate==CLIMATE_TEMPERATE) {
    replace grid_temperate_spr3981(3981, "temperate_groundtiles_gridline_8bpp.png") { template_groundtiles(0, 0, 1) }
}
if (climate==CLIMATE_ARCTIC) {
    alternative_sprites (grid_temperate_spr3981, ZOOM_LEVEL_NORMAL, BIT_DEPTH_32BPP, "arctic_groundtiles_gridline_bt32bpp.png") { template_groundtiles(0, 0, 1) }
}

Such behaviour is not possible for NewGRFs (cf. NFO specs) as alternative sprites are just one part of the definition of a single real sprite.

Some example NML and images:
redef-alternate.zip

Start by reproducing the behavior with the attached redef-alternate.zip example and inspect how alternative_sprites and conditional blocks are processed. Compare the result with the linked NFO specification; done means conditional alternative sprites follow their condition and nonsensical separated definitions are handled consistently with NewGRF semantics.