I have the Goo Peashooter and was able to level it up to 5. The same happened with Sling Pea.
All I did in each case was check that they had unlocked okay before moving on to do something else in the game.
I can't recall what I did when it was the Sling Pea, but with the Goo Peashooter I played the Pinata Party. Once I'd finished on each occasion I saw an exclamation mark on the Almanac that indicated a plant was ready to upgrade. When I went to the Almanac I found in each respective case that first the Sling Pea and then the Goo Peashooter plants were ready to upgrade.
I can assure you that I didn't cheat in any way. The upgrades just became available with no action being taken on my part.
There was a post from sub Reddit reporting a 'user friendly bug' which let players keep the 250 seeds used to unlock the plant. This was said to be possible only with certain actions after the plant is unlocked. But from your findings it appears that this bug has evolved to be even more user friendly. Shame it did not seem to happen on my IOS device.
So I guess my answer shoud be changed to: They got lucky!
@MAGICDAVESHROOM So you’ve got lucky too, I have not got enough seeds to unlock yet but if this happens with my lvl 1 plant collection then it will be a waste.
You do know that you can buy the seeds with gems like with shadow peashooter and slingpea?
Seeds for it are in the streak pinatas but not very many.