I started the map in question, in 1.12.2, then converted to 18w10a. It has successfully worked through 18w16a. I didn't try it with 18w19a, because I tried a test world first, and that was a MESS. When 18w19b came out just a bit later, I tested it, and it seemed to be working OK. However, it will not load my "Are We There Yet?" map (the one in question). Upon trying to open a copy of it, it hangs and never gets past this screen (See attached MC-18w19b-TryingToStart18w16aMap.png) (that was with a modified Soartex Fanver resource pack in place).

So I started testing in Vanilla, as follows:

(1) With vs. 1.12.2, I made a map named "Testing-18w19b-Starting:1.12.2" (Spawns in a Jungle)

(2) With vs. 18w16a, I made a map named "Testing-18w19b-Starting:18w16a" (Spawns in a Taiga)

(3) With vs. 18w19b, I made a map named "Testing-18w19b-Starting:18w19b" (Spawns in a Taiga)

(4) With vs. 18w16a, I opened a copy of "Testing-18w19b-Starting:1.12.2" and renamed it to "Testing-18w19b-1.12.2To18216a" (It opened correctly, in a Jungle.)

NOTE: I did not play or develop any of those maps in any way.

(5) With vs. 18w19b, I opened a copy of "Testing-18w19b-Starting:1.12.2" (It opened correctly, in a Jungle.)

(6) With vs. 18w19b, I opened a copy of "Testing-18w19b-Starting:18w16a" (It opened correctly, in a Taiga.)

(7) With vs. 18w19b, I opened a copy of "Testing -18w19b-1.12.2To18w16a" (It opened correctly, in a Jungle.)

(8) Seeing that all of these worked correctly, in Vanilla, I made a copy of my "Are We There Yet?" map, that was started in 1.12.2, played & developed somewhat, then changed to 18w10a, played & developed more, then opened, played, & developed in 18w16a. There had been some slight problems getting it to go from 1.12.2 to 18w10a, but from 18w10a to 18w16a, there were no problems whatsoever.

In Vanilla, it DID open with 18w19b. However, it began by "Preparing Spawn Area," THEN "Loading Terrain." Sure enough...all development was totally gone. It did seem to read the hybrid mix of the seed in 1.12.2 and 18w10a, but none of the development of mineshafts and villages, or any terraforming that had been done.

So...unless something drastically changes, I won't be able to take my "Are We There Yet?" map into 1.13.