Modded server but can only put stack size of 5k (custom stack size ) into furnace and not the asked 50k

"assets/prefabs/deployable/furnace/furnace.prefab": 10.0

Try to unload all other plugins with o.unload all command. Then load only ChestStacks and test different values for furnace and report the results.

ill have to make a test server to do that as its on a live server so itll take a while .. heres my plugin list so you can see if theres anything you tihnk may be causing the problem
01 "Active Sort" (1.0.4) by Egor Blagov (0.06s) - ActiveSort.cs
02 "Admin Radar" (5.1.2) by nivex (1.22s) - AdminRadar.cs
03 "Anti Spam Names" (1.3.0) by MON@H (0.00s) - AntiSpamNames.cs
04 "Arkan" (1.0.18) by Antidote (0.09s) - Arkan.cs
05 "Auto Lock" (2.4.1) by birthdates (0.01s) - AutoLock.cs
06 "Automatic Authorization" (1.2.4) by k1lly0u/Arainrr (0.09s) - AutomaticAuthorization.cs
07 "Auto Stash Traps" (0.2.9) by Dana (0.18s) - AutoStashTraps.cs
08 "BanScreen" (1.5.1) by MikeHawke (0.00s) - BanScreen.cs
09 "Blueprint Manager" (1.1.6) by Orange (0.01s) - BlueprintManager.cs
10 "Bradley" (1.1.5) by Ts3Hosting (0.06s) - Bradley.cs
11 "Building Workbench" (1.2.3) by MJSU (0.08s) - BuildingWorkbench.cs
12 "ChestStacks" (1.4.3) by MON@H (0.26s) - ChestStacks.cs
13 "Copy Paste" (4.1.27) by misticos (0.01s) - CopyPaste.cs
14 "Crafting Controller" (3.2.2) by Whispers88 (0.02s) - CraftingController.cs
15 "Death Notes" (6.3.5) by LaserHydra (0.28s) - DeathNotes.cs
16 "Discord Api" (0.0.2) by Pho3niX90 (0.00s) - DiscordApi.cs
17 "Entity Owner" (3.2.4) by Calytic (0.00s) - EntityOwner.cs
18 "Freeze" (2.3.0) by Wulf (3.21s) - Freeze.cs
19 "Gathering Manager" (2.2.75) by Mughisi (0.08s) - GatherManager.cs
20 "GetToDaChoppa" (1.0.5) by MikeHawke (0.02s) - GetToDaChoppa.cs
21 "Group Limits" (3.0.1) by misticos (0.00s) - GroupLimits.cs
22 "Heal" (3.1.0) by Wulf (0.00s) - Heal.cs
23 "Image Library" (2.0.58) by Absolut & K1lly0u (0.11s) - ImageLibrary.cs
24 "Godmode Indicator" (2.0.4) by 2CHEVSKII (0.00s) - GodmodeIndicator.cs
25 "Inventory Viewer" (4.0.4) by Whispers88 (0.01s) - InventoryViewer.cs
26 "Kits" (3.3.1) by Reneb (0.03s) - Kits.cs
27 "Lights On" (1.6.18) by mspeedie (0.35s) - LightsOn.cs
28 "Loading Messages" (1.0.6) by CosaNostra/Def (0.19s) - LoadingMessages.cs
29 "Loot Bouncer" (1.0.7) by Sorrow/Arainrr (0.08s) - LootBouncer.cs
30 "Magic Loot" (1.0.4) by collect_vood & Norn (1.63s) - MagicLoot.cs
31 "No Give Notices" (0.3.0) by Wulf (0.00s) - NoGiveNotices.cs
32 "OilrigSpawnFix" (1.0.1) by Steenamaroo (0.07s) - OilrigSpawnFix.cs
33 "Our Servers" (0.0.8) by Hougan / rostov114 (0.00s) - OurServers.cs
34 "PermissionsManager" (2.0.4) by Steenamaroo (0.04s) - PermissionsManager.cs
35 "PlayerAdministration" (1.6.2) by Pho3niX90 (0.04s) - PlayerAdministration.cs
36 "Playtime Tracker" (0.1.7) by k1lly0u (0.05s) - PlaytimeTracker.cs
37 "Plugin Update Notifications" (1.1.0) by Whispers88 (0.00s) - PluginUpdateNotifications.cs
38 "Quick Smelt" (5.1.3) by Iv Misticos (0.14s) - QuickSmelt.cs
39 "Quick Sort" (1.5.0) by MON@H (3.81s) - QuickSort.cs
40 "Raid Alarm" (0.2.1) by haggbart (0.03s) - RaidAlarm.cs
41 "Remover Tool" (4.3.27) by Reneb/Fuji/Arainrr (0.01s) - RemoverTool.cs
42 "SAMSiteAuth" (2.3.3) by haggbart (0.00s) - SAMSiteAuth.cs
43 "Server Armour" (0.6.20) by Pho3niX90 (0.02s) - ServerArmour.cs
44 "ServerInfo" (0.5.4) by FastBurst (0.04s) - ServerInfo.cs
45 "Server Rewards" (0.4.73) by k1lly0u (1.03s) - ServerRewards.cs
46 "SkinBox" (2.0.7) by k1lly0u (0.90s) - SkinBox.cs
47 "SmoothRestart" (1.2.5) by Fujikura/Visagalis (0.51s) - SmoothRestart.cs
48 "Stack Size Controller" (2.0.4) by Canopy Sheep (0.23s) - StackSizeController.cs
49 "Timed Execute" (0.7.4) by PaiN & misticos (0.00s) - TimedExecute.cs
50 "TimeOfDay" (2.3.4) by FuJiCuRa (0.00s) - TimeOfDay.cs
51 "Vanish" (1.5.4) by Whispers88 (0.00s) - Vanish.cs
52 "Welcomer" (1.5.6) by Dana (0.01s) - Welcomer.cs
53 "Wounded Screams" (2.2.3) by Death (0.00s) - WoundedScreams.cs

You need to do tests to find more info. At least you can try to unload ChestStacks and see if there is any difference. Also try to unload StackSizeController and see the difference.

Thanks for the help RESOLVED. Gave me ideas what to check. Turns out it was an older furnace sorter.