Changing the randomTickSpeed game rule to 0 disables most random world updates. These include:

• leaves decaying
• grass dying
• grass spreading
• fire spreading
• plants growing
• snow/ice melting
• etc.

As such, I'd expect that corals placed outside of water remain "alive" when the gamerule is set to 0, in a similar manner as everything above. But this doesn't seem to be the case. If you place any coral/coral fan/coral block outside of water, it's going to become dead in a matter of seconds regardless of how the game rule is set up.

The bug might not actually be that they die, but rather, that they don't respect the game rule at all. I'd also expect the opposite to happen, i.e. if I set a value larger than the default, that they'd also die faster than normal, which is also not the case.