Sand beaches generate when bordering Gravelly Mountains biomes

Confirmed in 1.13-Pre5

Confirmed for 1.13.1-pre1

Confirmed for 1.13.1

Can confirm in 20w51a. Since the coordinates provided in the description no longer demonstrate the issue, I've managed to find an area of where you can see this problem. Using the same seed, enter the following coordinates:

/execute in minecraft:overworld run tp @s -1631.49 98.01 843.78 -468.35 38.36

I am unable to reproduce this issue in 1.17.