This is also currently being tracked on Java MC-100949.

Since at least 1.18 (according to other reports) there has been an issue with shields not instantly blocking any incoming damage when activated. This causes there to be a slight visual and practical delay when the shield is used.

Steps to reproduce:

1. Spawn in a zombie and coax it into attacking you.
2. With your shield in hand, try and block any attacks.

Observed results:
There'll be a slight delay between when the shield is first activated to when it starts blocking any damage.

Expected results:
The shield should immediately start blocking any damage when used.