Reproduction steps:
1) Open user's attached project
2) Select 'astcTest' asset
3) Make sure that (in import settings):
-- Format (in Android section) is set to "RGB(or RGBA, both reproduce) Compressed ASTC 12x12 block"
-- Max Size is 8192
4) Click apply (if needed)

Expected result:
Actual texture should be shown (when Max Size is set to 4096 alike)

Actual result:
Texture in preview window/section (inside import settings) is purple.

Note:
This issue reproducible only in editor. If we'd apply texture to the material and render it on a plane using device (tested with Samsung Galaxy S7) - texture is rendered as expected (not purple).

Note 2:
If we apply texture to the material and render it using plane inside editor, in 5.5.0f1 (and 5.5.2f1 , didn't test between) renders it in white color (attached image);
If we'd do the same in 5.6.0b10 - Unity renders it purple as in texture's preview window.

Regression appeared in 5.5.0f1

Reproduced with:
5.6.0b10, 5.6.0b1, 5.5.2f1 , 5.5.0f1.

Unable to reproduce with: 5.4.4p4