Hi kenska,
Based on your description, I tested this problem in my environment Word 2016 with different file formats.
As shown in the following figure, I created a sample in Word and go to Developer- Properties, I checked >> 'don't allow editing' and/or 'don't allow deletion' settings. <<
Then I saved file and checked Save Thumbnail option.

I saved 5 types of file format, they are .doc, .docx, .docm, .dotm, .dotx with large icons.

As shown in the picture, this problem not appear with all file formats. As you said this problem is related to .dotm and .dotx.
As I can reproduce this problem, I suggest you give a feedback about this problem.
https://word.uservoice.com/forums/304924-word-for-windows-desktop-application
Regards,
Emi Zhang
TechNet Community Support
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