FastSuite version: 6.0.0
Placebo version: 9.7.0
NeoForge version: 21.1.133
KubeJS version: 2101.7.1-build.181

latest.log

so I would have to set kube to load after fast in the fml config?

Actually I can see the issue, tested it in a dev environment

fastsuite happens after the kubejs init, replacing the recipe manager (whose resources have been set from the kjs side)
fastsuite would need to specify that their inject happens earlier -> lower priority value (or kubejs needs to specify that they need to run later -> higher priority value)

might want to ask on the kubejs github / discord

[18Mar2025 15:00:58.198] [Render thread/WARN] [net.minecraft.client.gui.screens.worldselection.WorldOpenFlows/]: Failed to load level data or datapacks, can't proceed with server load
java.util.concurrent.ExecutionException: java.lang.NullPointerException: Cannot invoke "dev.latvian.mods.kubejs.core.ReloadableServerResourcesKJS.kjs$getServerScriptManager()" because "this.kjs$resources" is null

they seem to initialize some fields added by them to the RecipeManager from here:
https://github.com/KubeJS-Mods/KubeJS/blob/47386b68ecace68e394af3bbcfd20b2e53f00b4d/src/main/java/dev/latvian/mods/kubejs/core/mixin/ReloadableServerResourcesMixin.java#L45
(and with the latest FastSuite update this either doesn't happen anymore or it happens too late)

i wouldn't expect that to affect mixin application tbh (when they use default priority)