Can you post a picture of the brown mark?
I have had the same in the past but it was the cheap Chinese tape bought on eBay (never again)
I have the tape from mdp (blue 66m) no issues except it doesn't fit my tape dispenser.
Gold tape from inkexperts no issuesI did also buy 20 rolls from somewhere else but can remember for the life of me where and these were fine too.
The eBay stuff left a residue behind which does come off if you apply a little wd40 when he mug is cold and then wash thoroughly clean.
If your mugs don't clean off it may be something else?


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