Ok, this is a bit weird for me. Maybe I’m just used to the banjo but I don’t remember this happening to my guitar either. When my mandolin goes out of tune, most generally the A sting is the worst…it goes sharp. I have a tuner and re-tune frequently. The mando is kept indoors with consistent temp and humidity. Is this a thing for mando’s that I’m not aware of?
My brother’s mandolin was always going sharp. My fiddle has been going flat except for the A string which goes sharp. I’d check your nut, sometimes when you tune up your mando, (or any instrument) the string can get stuck in the nut, and the when you leave your instrument to sit (or sometimes when you play it) it’ll release the string which will take it sharp
Gunnar hit the nail on the head. Sometimes the nut slot is a bit snug and can pinch the string. Loosen the string and pull it out of the nut slot. Then sharpen a pencil and mark the inside of the nut slot with the sharpened pencil lead. That deposits graphite inside the slot which kind of acts as a lubricant to make the string slide through more smoothly.
Put the string back in, tune it up and see if the problem is resolved.
Good idea! I will try that.