... treatment protocol falls far, far short of acceptable, ...

[sitifan]

Again, as with obesity, if we cannot acknowledge that the prevailing treatment protocol falls far, far short of acceptable, then we will continue to be powerless to help those suffering. (My bold.)
Source: https://www.careforcancer.org.za/wp...anding-of-a-Medical-Mystery-by-Jason-Fung.pdf

Is the phrase in bold grammatically correct?

 

[jutfrank]

Yes.

fall short

1. to fail to reach an amount or standard that was expected or hoped for…

dictionary.cambridge.org

 

[sitifan]

What is the object of the preposition "of"?

 

[jutfrank]

What is the object of the preposition "of"?

I presume you're asking about meaning, not grammar, which is the right question to ask here. The argument of the preposition 'of' in 'fall short of' must denote a conceptual point. In this case, it's the point below which the standard of treatment is considered unacceptable.

 

[Annabel Lee]

What is the object of the preposition "of"?

Again, as with obesity, if we cannot acknowledge that the prevailing treatment protocol falls far, far short of acceptable, then we will continue to be powerless to help those suffering.

I think "acceptable" can be treated here as a truncated version of a noun clause like what is acceptable, what is considered acceptable, etc., which functions, unlike the adjective itself, as the "true" object of the preposition.

The protocol falls far short of what is acceptable.​