The Glowing effect has difficulty showing through more than a few blocks.

This seems to be related to the amount of air blocks in between the player and the entity with glowing, as breaking blocks not directly in the line of sight of the glowing entity can still cause the glowing entity to be visible through blocks.

This occurs both vertically and horizontally and seems to happen when there at least 10 blocks between the player and entity with glowing.

It is worth noting that once the air block that allows the glowing entity to be seen is uncovered, the entity can then be seen through blocks from seemingly any distance.