I personally do not recommend carrying 1911 with the hammer back and the safety off. Why woukd you want to? If for some reason your gun had some kind of major mechanical failure, the thumb safety is your backup. I suppose the thumb strap would stop the hammer from falling all the way. Besides that it sounds as if the holster is not functioning as intended. Does your 1911 have an ambidextrous safety? I have 4 1911s. None of them have the problem you seem to be having. Holsters should not disengage your safety, this is unusual. PM me and I may be able to help diagnose your problem. If you need holster selections, PM me and I'd be happy to offer suggestions.
Edit: Does it have a regular thumb safety or an extended thumb safety?