While playing on the current release (1.16.3) on a Spigot server (using Geyser), I was building in the nether and was attacked by a ghast. It hit my build, which is constructed of nether brick and white concrete, and, to my surprise, the concrete broke. I looked at the minecraft wiki and as I suspected, the ghast's fireball should be weaker than concrete's blast resistance, so it should not have broken.

I then decided to test this with my friend, and we've come to the conclusion that when a fireball hits netherrack, a surrounding concrete block will break. It will, however, stay intact if the fireball hits the concrete instead, as expected. We filmed an example of our test, which I've attached (note there is no audio in the clip due to size limitations)

Steps to replicate this bug: 

1. Make a build using netherrack and concrete 
2. Entice a ghast to shoot a fireball at the netherrack in your build. Similarly, you can summon a fireball with commands 
3. The concrete will uncharacteristically break if the netherrack is hit