Force country code on known numbers to send an SMS
I confirm that the problem persists on a pixel 3a. But, it is kind of reversed. In my case it is a US number and the SMS messages are linked to the number with the country code. But, within the US the messages must be sent without the country code. I find the same as the OP, there is nothing I can do to prevent the country code from being added. Delete the SMS messages, delete the phone book entries. Android is smart enough to always link the number I enter with the old entry that included the country code, recalling it from oblivion. But, it is too dumb to know that the message must be sent without the country code.

Also the answers given are missing the point. There is nothing we can do to control whether the country code is added. You can delete the contacts or edit them any way you like, the original form of the number will always be retrieved, with the result that you cannot text that number.