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Someone was writing their own save system - maybe T-Bone or 3-Com - but it would be loads of work no doubt.
The text is just sprites. There should be a folder called fontbank in your exported games folders, which contains the files needed for your game. Check they are there.
Others have reported issues with the save game system not working very well.
So if it all works until Saving the game, or starting a Saved game from the menu, then it is just another example of the save system being bugged out.
So perhaps report it on github and hope it gets fixed one day soon
Preben is currently working on bugs for classic so you may just get a solution.......
Maybe message him directly or send him an email
OldFlak....
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