WoWDB

All I know for sure is that the Bigger Fish to Fry achievement is required to find this quest item for the Underlight Angler artifact. Probably also need to be 110 and have 800 Fishing skill.

Regardless, this bugger can be hard to find. Doesn't matter if you log out and back in or where you fish (although pools are probably better than open water as usual), just keep at it. Like everything else in Legion, finding it is random.

I can confirm all of the above.  As for how long it took me to fish up Luminous Pearl: it was about a half-hour (factoring in travel time to fishing pools).  There is very good density of fishing pools in the rivers southwest of Moon Guard Stronghold in Suramar.