21w11a has a data version of 2703, which is also shared by combat test 7b. This is an issue as combat test 7 is forked from 1.16.2, and so no warning is shown when a combat test 7b world is loaded in 21w11a which should be an incompatible version.

This isn't really a resolvable issue but future snapshots need to jump up a few numbers to avoid this clash. The next free data version is 2708.