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No versions of TextPad, right back to the first, have had their own keyboard handler for the numeric pad. Instead, they rely on Windows to handle it and send TextPad the appropriate messages. Only if Num Lock is on does Windows send characters corresponding to the character code value typed while the Alt key is held down. Some Microsoft apps, such as Edge and the new version of Notepad, don't need Num Lock to be on, while others such as MS Office apps, Visual Studio and PowerShell behave the same as TextPad.

Have I misunderstood the issue here?

The theory exposes a strange subtlety in how various Windows apps handle the number pad, especially if applying Alt codes. TextPad, like other Microsoft apps such as MS Office and Visual Studio, relies on Windows' default behavior, which requires Num Lock to be enabled for Alt codes to work. The difference in behavior between apps such as Edge and the new Notepad shows a shift in Microsoft's strategy, maybe indicating an evolving standard for how input is processed across different software settings.

The Num Lock requirement has been eliminated in release 9.5.2 and 8.19.2.