I looked through the answers and I didn’t see what I’m about to say but I’ll keep it short because there’s no need to repeat certain things already stated.
I think what is most significant is that mix is used in conjunction with the kingdom of Heaven but hid is used in conjunction kingdom of God.
The two are not the same. What is different is not the same. The kingdom of Heaven is visible to all but the kingdom of God is not visible to all, therefore it is hidden.
##Notice the distinctions##
“And as you go, preach, saying, 'The kingdom of heaven is at hand.' Heal the sick, raise the dead, cleanse the lepers, cast out demons. Freely you received, freely give.” Matthew 10:7-8 NASB
These are all outward signs, visible to those born again and those not born again alike
This list is repeated to John the Baptist’s disciples and the word in the Greek is significant
“And He answered and said to them, "Go and report to John what you have seen and heard: the BLIND RECEIVE SIGHT, the lame walk, the lepers are cleansed, and the deaf hear, the dead are raised up, the POOR HAVE THE GOSPEL PREACHED TO THEM.” Luke 7:22 NASB
This word ειδετε means you can see the things with your physical eyes as well as mentally depending on the context.
Now contrast this with the kingdom of God.
“Jesus answered and said to him, "Truly, truly, I say to you, unless one is born again he cannot see the kingdom of God."” John 3:3 NASB
The Greek ιδειν is important in that it means to perceive with the physical eyes or perceive with the mind. Therefore you can do neither for the kingdom of God unless you are at the very least born again. I don’t want to go down this trail of explaining the kingdom of God and who and when it can be seen.
The fact that mix is used in one instance and hid in another is deliberate and consistent with the subject matter, kingdom of Heaven or kingdom of God respectively.
I have addressed the question, I don’t see the need to address a question that is not asked, namely what’s the difference between the kingdom of Heaven and the kingdom of God. If this doesn’t suffice please let me know.