Bee nests are not naturally spawning in worlds created in a previous version

I had the same issue with my world I have explored very far, you just need to go to new chunks, I went about 20k blocks away from my base and was able to find some to bring back. 

Try finding bee nests in unloaded chunks that you didn't explore while you were playing 1.14.4, perhaps? Bee nests can spawn in plains, sunflower plains and flower forest biomes.

Same problem on my Realm Server: I tried to generate over 500 new chunks, but no bee nest spawned.

I went too far, but they still don't spawn :/

probably just unlucky, each tree has a 5% chance

I have a realm that was generated several updates ago.  I have the newest update and have gone to several plains biomes and seen hundreds of trees, but no bees or beehives.  I went really far and checked chunks that I've never been to.  Once again, still no bees.

I updated my world from 1.14.4 to 1.15 and then to 1.15.1, and when I went exploring I found a bee nest quickly.

My understanding is that realms run on the latest version and that's why they don't provide an option to update to the next version (it's automatic).  Plus, my launcher says I'm running 1.15.1

I went 3,000 tiles away from where I usually adventure and was able to finally find bees and their nests in my realm.  It appears that you have to go to chunks that have never been loaded...

Currently, as with all features that generate in the world, in order to get the new features you must generate terrain that has never been generated in previous versions.