This is true a lot in games - often the "debug" version of a game is literally too slow to reproduce issues, so you have to work with the optimized builds.
...even if the actual debugging would be much easier with a debug build.
Understood. However even then I would hope that games have unit tests and other module tests that can run just a subset of code and not the entire game. When I'm writing code I want to know it builds and the basics work before I run the entire program.
I work on seasonal programs. I can only do fully realistic tests in the month of April (depending on the weather it shifts a few weeks) , meaning my window to test is closed for the next 11 months. I have learned to find ways to run my code in less realistic situations.
...even if the actual debugging would be much easier with a debug build.