Siri asks me which number to send a text to: home, work or mobile

I have an iPhone X and started encountering a problem while on iOS11, and it has continued with iOS12.

With SOME contacts, when I tell Siri to 'send [NAME] a text message', she asks "which number: home, work or mobile" and then displays all those numbers. With other contacts (even if they have multiple phone numbers), she just knows to send to their mobile number. I get the same home/work/mobile response from Siri if I ask 'send an iMessage to [NAME]'.

It doesn't happen with all contacts, but it happens consistently with certain contacts, and I don't understand how or why those contacts are different.

I have tried an instruction such as 'Siri, always use [NAME]'s mobile number when sending a text message', but she doesn't understand that, and then goes off on a tangent. FWIW, I get the same responses from Siri whether summoning her from my iPhone or Watch.

Any ideas or help here? Thanks!