I have the following strange problem with the GoG downloader:
I bought Rollercoaster Tycoon a few weeks ago and downloaded it.
On two separate occasions since then, I have added a new game to the downloader, and it has deleted my complete Rollercoaster Tycoon download, and started re-downloading that game from scratch without my telling it to, while putting the game I told it to download at the end of the queue.

Is there any solution to this?
It's really annoying that I've had to download Rollercoaster Tycoon 3 times because the downloader keeps deleting it.

BTW, I'm using the Opera browser.

That suggests Rollercoaster Tycoon's installer has been updated twice in a few weeks, which seems unlikely, but possible I guess.
Completing the third download of that file now, and going to both remove it from the queue and mark the file read-only.

Do you see this problem with other browsers?

Opera has various compatibility issues with all sorts of things for whatever reason. This may be one of them.