Do you have a shower with a shut off at the spray head?
If you do and shut off at the sprayer and not at the temp control valve you can get cross feed of hot to cold piping.
Common occurrence on RVs with an outdoor shower.
Otherwise I agree best to follow and diagram your piping to understand possible causes.
Is this something new? Always been that way? Or boat new to you?
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Don
2008 MS 34 HT Trawler
"Bacchus"
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