That sounds like either a Search option or a Smart Folder for a specific date range was selected. It could also be your Entries were sorted by something besides date.
You can tell if a Search option is active by looking just under the Search field: In Light Appearance mode, a blue description of the current Search options means at least one option is active; in Dark Appearance mode, the description is white regardless of the selected options but the text will be in bold if at least one option is active.
You can tell if a Folder or Smart Folder is selected by looking at the Accounts sidebar, where you'll see a blue or purple folder is highlighted if one or the other is selected. You can also look at the title of the window, where the name of the selected Account will be followed by a comma, a space, and the name of the selected Folder or Smart Folder.
To make sure your Entries are sorted by date, click the word
Date at the top of the column with all of the dates once or twice.
If you ever get into a situation where it seems like new Entries don't appear, please reach out to
support@splasm.com and we'll be happy to help you sort it out!