Holes in stamps disappearing

Hey! I’ve been having this problem for around a year and have struggled getting any help with it - I didn’t realise there was a forum I could post in! Hoping someone can help.

Sometimes when creating stamps, gaps fill in. For example, the gap in the letter “e” will no longer be there, so it will be a solid fill. I doubt it’s anything to do with the size of the hole, because often smaller holes remain and are fine. Also I use the same image in Fusion 360 and Tinkercad to create the STL and it slices & prints fine.

Anyone got any suggestions on how to fix this?
Thanks!

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