OOM: Preparing Spawn Area - Stuck at 3% (if remove files in region)

Watching log file, on server, starts normally, and gets to "Preparing spawn area: 3%".  Repeats this every 1-2 seconds, until the Java Heap Size caused a crash.

For each new snapshot, I delete all but the 4 region files for my world, where myself and a few friends are building/playing. I have been doing this since 18w08b, and not run into any issues.

When I got the error above, I went back and restored the full server files from 18w20c, and started the server again, and it loaded just fine.

Appears to be an issue with generating a new chunk... with everything restored to 18w20c, I loaded the game up just fine, then headed out to unexplored terrain, to generate a new chunk, and this was the crash:

crash-2018-05-23_10.25.45-server.txt

---- Minecraft Crash Report ----
// Who set us up the TNT?

Time: 5/23/18 10:25 AM
Description: Exception generating new chunk

java.util.concurrent.ExecutionException: java.lang.OutOfMemoryError: Java heap space
 at java.util.concurrent.CompletableFuture.reportGet(CompletableFuture.java:357)
 at java.util.concurrent.CompletableFuture.get(CompletableFuture.java:1895)
 at sj.c(SourceFile:129)
 at sr.a(SourceFile:84)
 at ss.c(SourceFile:143)
 at sk.i_(SourceFile:223)
 at net.minecraft.server.MinecraftServer.w(SourceFile:694)
 at rv.w(SourceFile:365)
 at net.minecraft.server.MinecraftServer.v(SourceFile:627)
 at net.minecraft.server.MinecraftServer.run(SourceFile:532)
 at java.lang.Thread.run(Thread.java:748)
Caused by: java.lang.OutOfMemoryError: Java heap space
 at it.unimi.dsi.fastutil.ints.IntArrayFIFOQueue.resize(IntArrayFIFOQueue.java:175)
 at it.unimi.dsi.fastutil.ints.IntArrayFIFOQueue.expand(IntArrayFIFOQueue.java:189)
 at it.unimi.dsi.fastutil.ints.IntArrayFIFOQueue.enqueue(IntArrayFIFOQueue.java:201)
 at cbm.a(SourceFile:142)
 at cbm.a(SourceFile:146)
 at cbm.a(SourceFile:130)
 at cbp.a(SourceFile:62)
 at tb.a(SourceFile:23)
 at sx.a(SourceFile:34)
 at bqc.a(SourceFile:87)
 at td.a(SourceFile:59)
 at td.a(SourceFile:23)
 at agg$a.a(SourceFile:143)
 at agg$a$$Lambda$1434/1510947075.apply(Unknown Source)
 at java.util.concurrent.CompletableFuture.uniApply(CompletableFuture.java:602)
 at java.util.concurrent.CompletableFuture$UniApply.tryFire(CompletableFuture.java:577)
 at java.util.concurrent.CompletableFuture$Completion.exec(CompletableFuture.java:443)
 at java.util.concurrent.ForkJoinTask.doExec(ForkJoinTask.java:289)
 at java.util.concurrent.ForkJoinPool$WorkQueue.runTask(ForkJoinPool.java:1056)
 at java.util.concurrent.ForkJoinPool.runWorker(ForkJoinPool.java:1692)
 at java.util.concurrent.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:157)

A detailed walkthrough of the error, its code path and all known details is as follows:

---------------------------------------------------------------------------------------

-- Head --
Thread: Server thread
Stacktrace
 at java.util.concurrent.CompletableFuture.reportGet(CompletableFuture.java:357)
-- Chunk to be generated --
Details:
 Location: 78,-63
 Position hash: -270582939570
 Generator: brt@7850dce9

Stacktrace:
 at sj.a(SourceFile:173)
 at sj.c(SourceFile:132)
 at sr.a(SourceFile:84)
 at ss.c(SourceFile:143)
 at sk.i_(SourceFile:223)
 at net.minecraft.server.MinecraftServer.w(SourceFile:694)
 at rv.w(SourceFile:365)
 at net.minecraft.server.MinecraftServer.v(SourceFile:627)
 at net.minecraft.server.MinecraftServer.run(SourceFile:532)
 at java.lang.Thread.run(Thread.java:748)

-- Affected level --

Details:
 Level name: world
 All players: 1 total; [sm['DonkBloodfist'/140, l='world', x=1239.73, y=140.36, z=-877.69]]
 Chunk stats: ServerChunkCache: 582 Drop: 0
 Level seed: -2079448837012124585
 Level generator: ID 00 - default, ver 1. Features enabled: true
 Level generator options: null
 Level spawn location: World: (-590,81,-693), Chunk: (at 2,5,11 in -37,-44; contains blocks -592,0,-704 to -577,255,-689), Region: (-2,-2; contains chunks -64,-64 to -33,-33, blocks -1024,0,-1024 to -513,255,-513)
 Level time: 133058642 game time, 24174213 day time
 Level dimension: 0
 Level storage version: 0x04ABD - Anvil
 Level weather: Rain time: 71652 (now: false), thunder time: 97339 (now: false)
 Level game mode: Game mode: survival (ID 0). Hardcore: false. Cheats: false

-- System Details --

Details:
 Minecraft Version: 18w21a
 Operating System: Linux (amd64) version 4.4.0-121-generic
 Java Version: 1.8.0_151, Oracle Corporation
 Java VM Version: Java HotSpot(TM) 64-Bit Server VM (mixed mode), Oracle Corporation
 Memory: 1033587584 bytes (985 MB) / 2044198912 bytes (1949 MB) up to 2044198912 bytes (1949 MB)
 JVM Flags: 2 total; -Xms1536M -Xmx2048M
 Profiler Position: N/A (disabled)
 Player Count: 1 / 20; [sm['DonkBloodfist'/140, l='world', x=1239.73, y=140.36, z=-877.69]]
 Data Packs: vanilla
 Is Modded: Unknown (can't tell)
 Type: Dedicated Server (map_server.txt)