No offence but we Korean sometimes sounds wrong, too. As you know, not all native speakers sound always right. Probably sometimes when we say Gamsahamnida, our tongue can be twisted wrong way. haha. that's why it could be heard like that way. But right way is Gamsahamnida, as we all know. But sometimes some people say Gamsahamda as well for fun. hope it helps you

The actual pronunciation in fast speaking is "감사함미다". It is because "합니다" is difficult to pronounce fast.

in fact,you can try ro say it quickly,you'll c u did the same thing,like "갑사합미다."