What is a word or phrase for something that, though it ought to be unbelievable, you can believe is actually true?

An example would be President Trump's tweets where when you're told about them your first instinct is to believe that can't possibly have been tweeted. But you nevertheless accept that it probably did happen.

Me: Can you believe Trump tweeted about eating his boogers?
You: At this point, yeah. Yeah I can.

Solution 1:

"It beggars belief."

beggar belief​ [phrase]

used for expressing shock at something that is extremely difficult to believe or describe

• His attitude simply beggars belief!

[Macmillan]

Obviously, the shock at observing the event or state entails that the event etc is actually real.

As other dictionaries state, this can also be used of totally unbelievable claims or theories, and is more common in the UK than the US.

........

A noun for an [almost] unbelievable event (usually one with good results) is, of course, a miracle. But we often use "Unbelievable!" "Incredible!" and so on with the watered-down "I can hardly believe it!" sense.