If you are having disconnect issues and you want help with figuring out how to stop it from happening again you should post in the technical support forum instead of the bug report forum or you could create a support ticket.
Unfortunately all disconnections and game crashes counts as leaving the match and will be penalized appropriately. The game server has no way to discern a legitimate disconnection or game crash from one that is forcefully caused by a dishonest player to bypass leaver penalties. You can learn about this policy here:
Based on your description it sounds like there are intermediate connection issues between you and your game client. The reason why you can disconnect in Competitive and not in other modes is that the game will backfill a replacement but keep you in queue for casual modes, but when you are assigned to a Competitive match, you must play that match.
Remember if you see the message, “Lost Connection to the Server” in the title screen or the message “Failed to Connect to Server” in yellow text in the text chat box, this means there is something that disrupted your connection to the game server and not a problem with the game server itself. There can be a variety of reasons this happens including those that don’t disconnect your computer from the Internet. There is great troubleshooting to start with that can most likely resolve most issues and I encourage you to check them out here:
As always if nothing in that guide works, please reply saying you completed these steps and additional advice can be provided.
Had the exact same issue, lost two competitive games without even joining in, total of 100 SR. Issued a ticket and the agent wasnt helpful at all. So I guess its just something you have to live with since its a flaw of the game, letting you search of a competitive game while not be able to join it.