A bizarre ending in the Jon Jones strip club vagina slap case -- the UFC star pled no contest to disorderly conduct ... but officials say they couldn't prove Jon actually did what he's accused of doing!!!

Remember, a waitress at TD's Eubank Showclub in Albuquerque told cops Jones had slapped her in the vagina during a wild April 19th night out with his NFL star brother, Chandler Jones.

The woman claimed Jon came up from behind her that night and put her in a "tight rear-naked choke hold" and then lifted her up in the air in what felt like a "wrestling match."

Once he put her down, the woman claims Jon slapped her right in the genitals.

Jones was initially charged with misdemeanor battery -- though he ADAMANTLY denied all wrongdoing and pled not guilty at the time.

Cops investigated the allegations and interviewed "numerous independent witnesses" -- but NOBODY could corroborate the woman's allegations.

In fact, cops say they tried to work with the accuser -- but she stopped cooperating with law enforcement during the investigation.

Ultimately, the Bernalillo County DA's office tells TMZ Sports, "There was not enough evidence to proceed with the petty misdemeanor charge of battery under New Mexico state law."

But, Jones isn't out of the woods ... somehow, officials say Jones agreed to plead no contest to the lesser charge of disorderly conduct and was sentenced to a 90-day deferment program, which we'll explain in a sec.

What we can't understand ... if no witnesses confirmed a crime took place and there's no longer a victim, why would Jones take a plea deal? We're trying to get in touch with his people for an answer.

As for the 90-day deferment program -- it essentially means the charge will be dropped altogether if Jones adheres to several conditions over the next 90 days, including:

-- no alcohol or drugs

-- no going to the strip club where the alleged incident took place

-- no contacting the accuser

We've reached out to Jon's camp for an explanation and we'll update once we hear back.