and you do NOT have a 12-month visa. There is nothing such. You received a "1-year Extension of Stay Permit". You are in Thailand on a "stay permit", not on a "visa". Check the stamp you got, what does it say? It says "extension of stay permitted until" and a date
because when you hold a VALID visa, Immigration will NEVER issue any other category of entry. You cannot legally hold two different visa or two stay permits at the same time