There are a whole host of reasons why a watch would run slow. Magnetism isn't usually one of them. When a watch becomes magnetized the hairspring coils become attracted to each other and 'stick'. What this does is shorten the active length of the hairspring and the watch will run fast. So I would assume some other reason is causing the watch to lose time.
The gentleman who repaired it for you should definitely fix it under warranty. I will offer a word of caution though, sometimes "watchmakers" can do more harm than good. If people aren't well trained and have a good general understanding of horological principles, it can turn into a nightmare and end up costing you a lot more in the long run.
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