Huh, I didn't know that! I usually just buy "magnet wire" atI have come across some that didn't break down even at temp full up.
At my suppliers some is declared "not solderable", esp. with high-temp wire it seems common.
Once had a reddish variety that didn't work that way, but i don't think color is an indicator.
Most of the wire i've seen can be tinned with your method tho..
You will notice if you pull the drop away if it is not properly tinned...