It's a blocker because it randomly causes the autotester to fail, such as here:
https://auto-tester.puremagic.com/show-run.ghtml?projectid=1&runid=3185054&isPull=true

The failing message:

0.066s PASS release64 std.experimental.allocator.building_blocks.allocator_list
testAllocator failed for BitmappedBlock!(64LU, 8u, NullAllocator, cast(Flag)true)
****** FAIL release64 std.experimental.allocator.building_blocks.bucketizer
core.exception.AssertError@std/experimental/allocator/building_blocks/bitmapped_block.d(112): Data must be aligned properly
----------------
??:? _d_assert_msg [0x71d728e]
??:? pure nothrow ref @nogc std.experimental.allocator.building_blocks.bitmapped_block.BitmappedBlock!(64uL, 8u, std.experimental.allocator.building_blocks.null_allocator.NullAllocator, 1).BitmappedBlock std.experimental.allocator.building_blocks.bitmapped_block.BitmappedBlock!(64uL, 8u, std.experimental.allocator.building_blocks.null_allocator.NullAllocator, 1).BitmappedBlock.__mixin2.__ctor(ubyte[]) [0x6d880b7]
??:? pure nothrow @nogc std.experimental.allocator.building_blocks.bitmapped_block.BitmappedBlock!(64uL, 8u, std.experimental.allocator.building_blocks.null_allocator.NullAllocator, 1).BitmappedBlock std.experimental.allocator.building_blocks.bitmapped_block.BitmappedBlock!(64uL, 8u, std.experimental.allocator.gc_allocator.GCAllocator, 1).BitmappedBlock.__unittest_L1344_C17_1().__lambda2() [0x6d8a33f]
??:? void std.experimental.allocator.common.testAllocator!(std.experimental.allocator.building_blocks.bitmapped_block.BitmappedBlock!(64uL, 8u, std.experimental.allocator.gc_allocator.GCAllocator, 1).BitmappedBlock.__unittest_L1344_C17_1().__lambda2()).testAllocator() [0x6d89af6]
??:? void std.experimental.allocator.building_blocks.bitmapped_block.BitmappedBlock!(64uL, 8u, std.experimental.allocator.gc_allocator.GCAllocator, 1).BitmappedBlock.__unittest_L1344_C17_1() [0x6d86284]
??:? void std.experimental.allocator.building_blocks.bucketizer.__modtest() [0x6d81c38]
??:? void test_runner.doTest(object.ModuleInfo*, ref core.runtime.UnitTestResult) [0x5dde6d4]
??:? core.runtime.UnitTestResult test_runner.testModules() [0x5dde60b]
??:? core.runtime.UnitTestResult test_runner.tester() [0x5dde4df]
??:? runModuleUnitTests [0x71d7e0a]
??:? void rt.dmain2._d_run_main(int, char**, extern (C) int function(char[][])*).runAll() [0x71ec96c]
??:? void rt.dmain2._d_run_main(int, char**, extern (C) int function(char[][])*).tryExec(scope void delegate()) [0x71ec8f7]
??:? _d_run_main [0x71ec865]
??:? main [0x5dde383]
??:? start [0x5dde353]
??:? 0x0 [0x1]