I dunno, are you sure you got legitebly working dlc in the right folder with matching region for both game and dlc?
The game and the DLC share the same region and I put the latter in "ux0:/pspemu/PSP/GAME". This DLC works in PSP's and PS Vita's/PS TV's standalone ARK-4 but the latter often crashes in the "#ChovyProject" part with a C1-2858-3 error message after reinstalling PS Vita's/PS TV's standalone ARK-4 via the new FasterARK method and running it, and when it doesn't, it often crashes when running PSP ISOs with a C1-2858-3 error message. I have this issue if PS Vita's/PS TV's standalone ARK-4 "Memory Stick Speedup" is set to "Always" (it is by default). I'll set it to "Disabled" and delete Adrenaline and its plugin installed via AutoPlugin II. This method unlike the previous FastARK one doesn't install the Sasuke vs. Commander game if no PSN game/demo is installed and doesn't require a PSN game/demo (like Ape Quest's demo) to be installed before being run as well, at least. It's thanks to this "C1-2858-3" error message I'm using Adrenaline. I only have problems loading this PSP game's DLC in Adrenaline, not when loading other PSP games' DLC with it, when loading this PSP game's DLC with ARK-4 and when loading other PSP games' DLC with the latter.
EDIT: I used a tool called ISO Tool to update LittleBigPlanet's ISO and it successfully loaded its DLC in Adrenaline. I missed the part in the downloaded DLC's "Readme" which says to use ISO Tool in PSP's PRO-C CFW to install the provided update out with the DLC, though I'm on the PS Vita and PS TV and not on the PSP (and I also have ARK-4 CFW installed on the latter, not PRO-C, and it successfully loads LittleBigPlanet's DLC without updating its ISO [I deleted LittleBigPlanet's ISO, downloaded it again and loaded the DLC without updating the ISO with ISO Tool], so ISO Tool isn't needed and as a result I deleted it).
EDIT 2: It seems PS Vita's/PS TV's standalone ARK-4 crashing issue was finally fixed and "Memory Stick Speedup" may be set to "On" (it is by default. "Always" no longer exists, only "On" and "Off").
"OverClock" was renamed to "CPU Clock in Game" and is set to "OverClock" by default (I set it to "Auto" on the PS Vita to not wear its battery and kept it as "OverClock" on the PS TV since it doesn't have a battery), "Use Extra Memory" is set to "Auto" by default, "Memory Stick Speedup" is set to "On" by default (like I said) and "Inferno Cache" is set to "LRU" by default.
On the PSP, "OverClock" was renamed to "CPU Clock in Game" and "CPU Clock in XMB" and they're set to "OverClock" by default (I set them to "Auto" to not wear its battery), "Use Extra Memory" is set to "Auto" by default, "Memory Stick Speedup" is set to "On" by default and "Inferno Cache" is set to "LRU" by default.
EDIT 3: I installed PS Vita's/PS TV's standalone ARK-4 new NoPspEmuDrm (which bypasses all PSPEmu DRM Checks so that digital PSP and PS1 game backups can be played or those that are legitimately owned but on another PSN account. This plugin is needed for ARK-4 to work) and ef2uma (which according to
this, maximizes the amount of storage space that can be used simultaneously) plugins on the PS Vita and PS TV.
EDIT 4: XMB and "Custom Launcher"/PS Vita's/PS TV's standalone ARK-4 often don't find a new ARK-4 version when it was just released and I have to wait some time for them to do it or manually install it. According to what I read in ARK-4's GitHub repository, it seems the stable release server has to be manually updated and the person in charge of the server isn't always available exactly on release. Also, when scrolling games on PS Vita's standalone ARK-4, the console makes noise even with no sound.
EDIT 5: Pressing "SELECT" on the XMB does nothing for some seconds when returning from sleep mode, when exiting a game and after restarting the PSP or the "VSH MENU" so the latter doesn't appear. The PSP's "Memory Stick PRO Duo access indicator" blinks and only once it finishes I can access the "VSH MENU" (among other things).
EDIT 6: I installed the nploader_mod (which allows direct reading and loading of DLC [EDATA/PGD/SPRX] files in either their encrypted or decrypted form, so I don't need to reactivate my account and/or switch between them while using my purchased DLC from different regions. I can mix decrypted/encrypted files without problem too, since the plugin takes care of it) and npdrm_free_mod (which is a plugin to run official NPDRM PS1/PSP content without the need of a valid RIF license or act.dat file) plugins on PSP's ARK-4 and enabled them for both game and xmb only like said
here.
EDIT 7: Now, PSP's ARK-4 comes with the LEDA plugin enabled and with the bsd one disabled. The former is designed to provide compatibility with legacy software/homebrew that uses the 1.50 kernel and the latter makes a message show up for somes seconds when running games and homebrew and when exiting them via "PS" button saying:
"Starting BSoD
ERROR loading user module: 80020148"