I've run into situations where I've been just idling at the main screen and suddenly it gets all liney. When I then click on the home button, it pops up showing a red Bloom Dragon there, with the line effect around it.

I haven't kept count, but is this me getting a free level 100 red bloom dragon?

nope it doesn't give you a free dragon. i tried that out
just click on the home button again and again. you will see a dragon sometimes, but you won't get one free.

Its an easter egg. If you encounter a Bloom Dragon on you main screen, with sound turned on you´ll notice they have male Voice Over (though they are clrearly female) shouting stuff like "I too like you Knight Commander ! I will help you/support you" and stuff like that.
Its just a little trololo from the devps.
So yeah, trap dere dere dragons are a thing apparantly

Dang. Got my hopes up, because I've been running the heck out of the 80 stamina bloom dragon mission and those level 100s are just not showing up.

Dang it, I've got evolutions to evolute!

Originally Posted by Myrdin

(though they are clrearly female)

What gave you that impression?
I thought they were going for a stereotypical anime bishounen with them just to mess with the player.