All of our snails are dying :(

We are very new to aquarium-keeping. We got a tank and 5 ramshorn snails 3 weeks ago. They were doing ok! Then in the last few days, 4/5 have died.

We test daily and our PH got a little close to the "change water" zone but it didn't actually get there. Everything else was in the safe zone. We feed them fish flakes and cucumber. There were a ton of baby snails as well yesterday! I thought breeding would be a sign they're doing ok but I guess not.

We did add 2 mystery snails last week but they're peaceful.

Our filter works. We have plants in there. I don't know what else we're doing wrong???