So would I! I don't know, and I'm not sure anybody does. I was basing my suggestion on what FAL said (that the UK will continue to issue EHICs for some) and others' continued assertion that EU citizens' rights are protected by the withdrawal agreement - although it's not clear to me that the WA covers what happens after the end of the transition period. Also from the NHS website I sited previously:
"You'll still be able to access healthcare through EHIC for visits that begin after 1 January 2021 if you're either:
a UK state pensioner living in the EU before 31 December 2020
a UK student studying in the EU before 31 December 2020
a 'frontier worker' (someone who works in one state and lives in another) before 31 December 2020, for as long as you continue to be a frontier worker in the host state
an EU national living in the UK before 31 December 2020"
I am confident that you will not be affected, as you are contributing to the Spanish system and are covered therefore by Spain and will continue to be. But when you say "I think It's the country which issues the card which matters not the nationality of the holder ", I would tend to agree, except that it seems my Spanish friend will be eligible for a UK issued card, but as a UK citizen I won't be!