@ipentest
I can only reproduce this if the first thing I do after creating the notebooks and note is search for "Antidisestablishmentarianism". If I do nothing else but just re-enter the word and search again it works. Also I cannot reproduce this once I have previously created a notebook, any notebook, in Joplin with two stacked sub-notebooks. I could only trigger it again by using the same process but in a notebook with three stacked sub-notebooks. So it looks like Joplin just needs a little bit of time to adjust its indexing, as @jamedeus suggests, when deeper and deeper notebook "stacks" are created for the first time.
Hopefully your search is working now...