[Bug] Impossible to read all (old?) articles in a feed

[Massedil]

Reproduce:

I choose a feed with 68 unread articles from today 2021-10-14 15:34:00 to 2021-09-08 09:51:00 (I see it in the FreshRSS web app).

1. Click on the "Load more article" button:

GET /api/greader.php/reader/api/0/stream/contents/feed/801?n=50&xt=user%2F-%2Fstate%2Fcom.google%2Fread&likes=false&comments=false&r=n&client=android&output=json&ck=1634230279516

2. Click on the "Load more article" button again to get the last articles:

GET /api/greader.php/reader/api/0/stream/contents/feed/801?c=1634021558650966&n=50&xt=user%2F-%2Fstate%2Fcom.google%2Fread&likes=false&comments=false&r=n&client=android&output=json&ck=1634231123610

Actual:

The button "Load more article" is now "End of article" so I can't check for other unread article again.

Only 2 more article are loaded instead of the last 18.

If I read all the loaded article in the app, the main screen say there is 16 unread articles but can't load it.

The strange thing is that all articles from october 2021 are loaded, but not the articles from september.

Expected :

All 68 articles are loaded.

Screenshot of FreshRSS web app view :