When you find or use the /locate command to get to a Pillager Outpost, you will notice that the entire structure is completely absent. Pillagers can still sometimes spawn in these areas, as well as additional side structures.

Cages, tents, logs, dummy targets, and other additional decorations will still be generated, but the main tower is seemingly made up entirely of air blocks. This can be seen where the Pillager Outpost that is located in the water causes missing water source blocks, as shown in the screenshot below.

I am unsure of what causes this, because I have yet to see what causes the Pillager Outpost to not generate. Of all the Outposts I have found, both in Creative and in Survival, I have not found a tower that has spawned in successfully.

I have seen a similar issue on the bug tracker, but all the screenshots present did not have decorative Pillager stuctures present. Apologies if this is still considered a duplicate.