Certain blocks or entities don’t appear when looking through a spectating players head, or while spectating in 3rd person perspective. These include:

1.) Hostile and passive mobs (sometimes)
2.) Other players (sometimes)
3.) Water blocks
4.) Dropped items
5.) Player heads
6.) Shields on-hand
7.) Clouds
8.)Chests
9.)World Border
10.) Glass panes
11.) Arrows in blocks (sometimes)
12.) Ice

_{More things might be affected than are listed here}