I got stamped in once for 30 days instead of 12 months, thankfully I remembered your previous posts and checked. It caused a bit of commotion but thankfully they managed to correct it there and then.
Always pays to listen to the advice and check the entry stamp immediately as it definitely appears that it’s a common mistake by only been given 30 days, this happened to me and was promptly corrected by the officer when I pointed it out.
You still have to do you 90 day report, if you have an multiple entry visa you only need to to do a visa run after just before your 12 months so you get your second 12 months if you require it, but remember your second 12 months doesn’t carry they Multiple entry aspect so you would have to got to immigration to pay for your second year should you wish to leave Thailand