It's as the header says: When I connect to my modded server, the Blockpack is able to hold the full 254 item stack, but once a stack has more than 127 items, it turns invisible. No new messages appear in the server or client logs and it works just fine on singleplayer.

There is a difference in mod loadout between the two, but even running the server's mod list on singleplayer was fine. The bug only happens when connected to a server, it seems. The player data still shows the items in the amounts they should be, they are just invisible in the pack.

I've attached the latest.log for both and the most recent log that has the server's mod list. This is using the Luna's Origins modpack v. 2.3.4: https://www.curseforge.com/minecraft/modpacks/lunas-origins-modpack
latest.log
latest.log
2023-04-21-2.log