Thank you very much for your appreciation and for spotting the problem!
I'm sorry about the issue, I thoroughly tested the "expanded code" version of the game, but not enough the "compressed" one :'(
The problem was due to the old ugBASIC runtime; I rebuilt the game with the latest version of ugBASIC (1.17.1) and it now seems to work fine. You can download the fixed package Lader_BASIC10LinerContest_fix.zip from the project page: https://retrobits.itch.io/lader
Thank you again for your appreciation. Yes, I deliberately chose to keep the game fast and difficult to make it a bit challenging, especially since there are only 3 distinct screens.
However, on request by one user, on the newer versions available on retrobits.itch.io/lader it is possible to choose the desired game speed.