For what it's worth, I think this issue may now be fixed (17 March 2019). The trick is, don't trust the "copy" button.
Not knowing any better, today I created an SSH Key note. Yesterday, for the first time in my life, I had created an SSH key (using PuTTY).
Today I copied/pasted its public key (the contents of id_rsa.pub) into the "public key" field. Which, everybody agrees, works just fine.
Next, I copied/pasted its private key (the contents of id_rsa.ppk, which is the format PuTTY uses) into the "private key" field.
Having read all 5 pages of this thread, I also pasted it into my Notes field.
In the LP secure note, the Private Key field only shows the first line -- and only the first part of the first line, of course. It's a relatively small-looking field.
And if you press the field's "copy" button, you only get the first line of the key. Just like everyone has said.
However. If you go old-school -- put your cursor somewhere in the field, press CTRL*-A (select all), then press CTRL*-C (copy) -- then you get the whole darned thing.
Which you'll discover when you create a new blank document in, say, your NotePad++ editor, and press CTRL*-V (paste). Because then you'll get the full private key. Including the all-important line breaks.
*I'm a Windows guy, so that's what I use. If you're on other OSs, then of course use what you use.
So I think we're good on this, now. Just don't trust the "copy" button. Select-all, copy, then paste. No school like the old school.
Best wishes, all.