When an older world is upgraded to 1.18 pre-6, under specific circumstances a pillager outpost will generate but not spawn pillagers.

The cause is structure references for pillager outposts (Level > Structures > References > pillager_outpost) pointing to chunks that have not yet generated. When these chunks are loaded in the world in 1.18 pre-6, the structure will generate but the structure information is not saved in the chunk (Structures > Starts > pillager_outpost is INVALID).

I have attached a world save that reproduces the issue.

The structure is a pillager outpost at 80, 320. The corresponding chunk containing the outpost is NOT present in the save file (region 0,0 chunk 5,20) but adjacent chunks that reference it are present (eg: region 0,0 chunk 5,19). This adjacent chunk has a reference pointing to the chunk at 5,20 (Level > Structures > References > pillager_outpost: 05 00 00 00 14 00 00 00 hex decodes as 5,20).

If the chunk containing the pillager outpost is present in the save files, (the outpost has generated), the outpost will spawn pillagers as normal. It requires this edge case to produce a defective outpost.