Articles + uses Apple's default text selection, which can sometimes be unreliable. The behaviour (and issues) are the same as when you for example select text in Safari browser.

What usually works best is to:

1. Long press on an individual word
2. Adjust the selection handles afterwards

There will likely be some optimizations in an upcoming iOS platform update. Please report this issue directly to Apple for it to be prioritized:
https://www.apple.com/feedback/

On macOS, you can right click text to get a context menu to create highlights or notes. But this no longer works on macOS Monterey because of a system level issue that can't be fixed.

It kind of works again on Ventura, but shows all menu entries instead of only the current. Please note that "Look up" and "Translate" are not available on macOS because this feature is somehow not supported on that platform.

It is recommended to use the toolbar or keyboard shortcuts for highlights/notes:
https://www.article-reader.com/support/faq/#peripherals-shortcuts

Due to a bug in macOS 11.4+, layout rendering for some Mac Catalyst apps no longer works as expected when an external monitor is connected. While the correct content is initially displayed, it breaks the more you scroll or change a page.

According to Apple, a system-level fix is required. It should now work again with macOS 12, but unfortunately a patch for macOS 11 will not be released.

There is no workaround available and you will need to either update to macOS 12 or disconnect external displays. Please report this problem directly to Apple:
https://www.apple.com/feedback/