Undoing driver seat belt disengages transmission?

A few weeks ago, I backed car up into driveway and turned off. I wasn't able to start it for a while as the transmission light came on and gear light was flashing. Fuses, relay, fluid levels checked. After charging up weakened battery, car eventually started and got into gear and the transmission was fine. The other day was driving on highway at 70 mph. I took seat belt off to get something out of pocket. Seat belt alarm went off. After about 30 seconds, got seat belt back in and went to press gas and the engine revved like it wasn't in gear when it was. I had no forward power. Pulled car to side and wasn't able to start again. After a few mins was able to restart and transmission was fine. I heard there are issues when shutting car off when still in reverse and that may explain the first problem, but wondered if there is a safety check that when seat belt off for a period of time when at speed, does the transmission disengage as a 'safety feature' or there is an issue that needs to be examined?
thanks