Well, perhaps my native-speaker intuition is not working correctly, but I am not bothered by either of the following:
"To understand the issue, this is the key."
"This is the key to understand the issue."
Both sound perfectly fine to my (apparently flawed) ear.
[...] Is it not possible to say that to understand the issue is an infinitive phrase that modifies the key? I guess my Spanish mind doesn’t see why in this sentence “to” must function as a preposition and not as part of the infinitive “to understand.” [...]