This happens for every unit upgrade though the numbers depend.
Very easy to reproduce 100% of the time
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Place a man at arms.
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Create a trigger Modify Attribue Add 5 HP to man at arms
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Create a trigger Modify Attribue Add 5 HP to Militia
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Man At Arms now has 40/50 HP.
Expected Behaviour: Increasing Hit Points of Militia should not effect Man at arms
Additional Info: This only happens if you have added hit points to man at arms (This is why Step 2 is necessary)
More info:
Same scenario but instead Add 10 HP to militia
Man at arms now has 45/50 HP
Same again but instead Add 35 HP to militia
Man at arms now has 67/50 HP
Same scenario but instead Loop Timer 1 to Add 1 HP to militia
Man at arms now loses HP every second until it starts increasing exponentially shortly after he will just die without taking any damage.