Yes—this can be a film behavior, not necessarily a camera fault.

In theory, splitting one correct exposure into several smaller ones should add up the same. But film is not perfectly linear across all exposure levels. Very small partial exposures can fall on the low end (“toe”/heel) of the film’s response curve, where they build density less efficiently, so many small exposures may produce a darker result than expected.

There’s also reciprocity failure: at longer effective exposure times, film sensitivity drops and exposure no longer scales linearly. For multiple exposures, you need to consider the total exposure time across all exposures when checking the film’s reciprocity data. This is especially important with reversal film.

So your simple method works best only when each partial exposure stays within the film’s more linear response range and the total time doesn’t trigger reciprocity issues. If the image gets darker as exposure count increases, film response is the likely cause.