It is not a bug, it is feature and it works like that: Depending on how you fall down to the ground, you are able to make a shield/weapon block while you are getting up. For example, you should not be able to make a block if you fell down face downwards. Additionally, you are able to do it after a specific time of the getting up animation.(For example, getting up animation takes 4.5 seconds and you are able to make a block after 2.5 seconds passed). However, we will have a look at that times to adjust this feature. Thanks for reporting, enjoy the game!
Edit: It is not related with the cavalries, if a stray horse or a cavalry makes you fall down, still this feature is valid.