For it to work, I found that I have to place the landfill, add items, then remove it, then place it again. That way the landfill works.

Just tested in vanilla and it works as expected. Can you share your mod list?

I do seems to have the same problems with them. Sometimes after building them they just doesn't eat the items in them.
They never fix themself over time if it happens, and changing the mods values doesn't affect the bugged landfills. After rebuilding them multiple times they eventually get fixed and never gets broken again.

the save: https://drive.google.com/file/d/1LVht-loRicF3Jx6boDlxED2MoSX7qaI3/view?usp=drive_link
those landfills at the loading of that save have been bugged for 6-700h of gameplay, i'm pretty sure they never fix themselves without rebuilding.

the mods i'm using:
base 1.1.110 simhelper 1.1.6
aai-containers 0.2.11
aai-signal-transmission 0.4.9
alien-biomes 0.6.8
auto_sort_chests 1.0.1
ChangeMapSettings 5.0.1
Cleaned Concrete 1.0.4
flib 0.14.1
informatron 0.3.4
jetpack 0.3.14
Kux-CoreLib 2.11.0
ModuleInserterSimplified 1.1.4
nixie-tubes 1.1.5
robot_attrition 0.5.15
shield-projector 0.1.6
space-exploration-graphics 0.6.17
space-exploration-graphics-2 0.6.1
space-exploration-graphics-3 0.6.2
space-exploration-graphics-4 0.6.3
space-exploration-graphics-5 0.6.1
space-exploration-menu-simulations 0.6.8
ZCS-Trash-Landfill-Continued-Continued-Continued 1.1.0
aai-industry 0.5.22
Kux-BlueprintExtensions 2.3.2
ModuleInserterEx 6.1.2
Recipe Book 3.5.8
space-exploration 0.6.128
se-scanning-already-scanned-surface-recharts-it 1.0.2
space-exploration-postprocess 0.6.28