Betta Fish: My dwarf cory’s are dying

Hey everyone, I need some help. I have a heavily planted 10 gallon that’s been running for about 3 months. It currently has around 12 shrimp that are doing great and breeding like crazy. I originally had 3 dwarf corydoras (not pygmy, the other dwarf species — either hastatus or habrosus). They even bred in the tank and I saw a few tiny fry swimming around.

But recently the adults started dying one by one, and now I’m not sure what happened or if any fry survived.

Here are my parameters (API test kit):
• Ammonia: 0.25 (probably went up when one of them died)
• Nitrite: 0
• Nitrate: 0
• pH: 7.6

I’m confused because the shrimp seem totally fine, but the dwarf corys didn’t make it.

Something I noticed: my corys NEVER ate sinking wafers or pellets. They only ate crushed pest snails. But now the pest snails are basically gone, so I wasn’t crushing snails anymore. I’m starting to think maybe they were starving?

Is it possible they weren’t eating enough and then the small ammonia spike finished them off?
Or does this look like the tank wasn’t fully cycled even though plants keep nitrates low?

Any advice is appreciated. I just want to figure out what went wrong before I add any fish again