Error with item tags in advancements

The game won't accept advancements with a "#" item type in the player equipment condition, and therefore keeps previous data instead of updating when loading an advancement using a "#" item type.

I have no idea whether this is because Minecraft loads advancements before it loads item tags, or if it's because "#" isn't accepted by the player equipment condition in an advancement (Which would be weird, considering it works in other cases).

Here's the point in the output log that states the error:
Failed to execute reload java.util.concurrent.CompletionException: t: Non [a-z0-9_.-] character in namespace of location: #super_hard:armor at java.util.concurrent.CompletableFuture.encodeThrowable(CompletableFuture.java:273) at java.util.concurrent.CompletableFuture.completeThrowable(CompletableFuture.java:280) at java.util.concurrent.CompletableFuture.uniAccept(CompletableFuture.java:659) at java.util.concurrent.CompletableFuture$UniAccept.tryFire(CompletableFuture.java:632) at java.util.concurrent.CompletableFuture$Completion.run(CompletableFuture.java:442) at abg.a(SourceFile:71) at abg$$Lambda$2844/1041367517.run(Unknown Source) at uy.run(SourceFile:18) at amn.c(SourceFile:144) at amr.c(SourceFile:23) at net.minecraft.server.MinecraftServer.b(SourceFile:763) at net.minecraft.server.MinecraftServer.c(SourceFile:154) at amn.x(SourceFile:118) at net.minecraft.server.MinecraftServer.aZ(SourceFile:745) at net.minecraft.server.MinecraftServer.x(SourceFile:739) at amn.c(SourceFile:127) at net.minecraft.server.MinecraftServer.a(SourceFile:1339) at ws.a(SourceFile:22) at ws.a(SourceFile:53) at ws$$Lambda$3439/1581023858.run(Unknown Source) at com.mojang.brigadier.CommandDispatcher.execute(CommandDispatcher.java:262) at com.mojang.brigadier.CommandDispatcher.execute(CommandDispatcher.java:176) at da.a(SourceFile:260) at zv.c(SourceFile:1134) at zv.a(SourceFile:1121) at rd.a(SourceFile:36) at rd.a(SourceFile:9) at nk.a(SourceFile:21) at nk$$Lambda$4413/612291925.run(Unknown Source) at uy.run(SourceFile:18) at amn.c(SourceFile:144) at amr.c(SourceFile:23) at net.minecraft.server.MinecraftServer.b(SourceFile:763) at net.minecraft.server.MinecraftServer.c(SourceFile:154) at amn.x(SourceFile:118) at net.minecraft.server.MinecraftServer.aZ(SourceFile:745) at net.minecraft.server.MinecraftServer.x(SourceFile:739) at amn.c(SourceFile:127) at net.minecraft.server.MinecraftServer.w(SourceFile:724) at net.minecraft.server.MinecraftServer.v(SourceFile:676) at net.minecraft.server.MinecraftServer.a(SourceFile:256) at net.minecraft.server.MinecraftServer$$Lambda$3968/2041123334.run(Unknown Source) at java.lang.Thread.run(Thread.java:745) Caused by: t: Non [a-z0-9_.-] character in namespace of location: #super_hard:armor at uh.<init>(SourceFile:38) at uh.<init>(SourceFile:46) at bo.a(SourceFile:126) at bb.a(SourceFile:72) at be.a(SourceFile:167) at be$b.a(SourceFile:373) at be$b.a(SourceFile:348) at ci.b(SourceFile:45) at ci.a(SourceFile:17) at ab.a(SourceFile:36) at ab.b(SourceFile:47) at w$a.a(SourceFile:318) at us.a(SourceFile:45) at us$$Lambda$3789/220692581.accept(Unknown Source) at java.util.HashMap.forEach(HashMap.java:1280) at us.a(SourceFile:42) at us.a(SourceFile:26) at abf.a(SourceFile:13) at abf$$Lambda$2652/1476548395.accept(Unknown Source) at java.util.concurrent.CompletableFuture.uniAccept(CompletableFuture.java:656) ... 40 more

Before you ask, yes, i have checked that the custom item tag is a valid tag, and yes, the game does accept the advancement if i swap out "#super_hard:armor" with, say for example, "minecraft:stick". Vanilla item tags won't be accepted either.