I got second F7Pro, first had damaged steel vibrations. Now on the second watch I realized, that there is dust and something like rubber glue inside the HR sensor... I don't know why I have so much luck for this pieces. Don't know if it could affect HR measurements.
I contacted Garmin service center and they told me, that there definitely shouldn't be any dust or something in the sensor and that they can't guarantee me the waterproof of the watch and…
That's not going to affect anything. Each sensors records just the total amount of light entering it, so minor dust or even scratches is not going to affect it, since their surface area is minuscule compared…
Of course you alone decide what to do, but if I were you, here's what I would do: You have a new watch functioning properly and it's under warranty. Several people here (including me) have given you reasons…
Of course you alone decide what to do, but if I were you, here's what I would do: You have a new watch functioning properly and it's under warranty. Several people here (including me) have given you reasons why small amount of dust won't affect HR readings. And a Garmin representative has said your watch should be ok. So use your watch and enjoy it. It's not a piece of jewelry, it's a device, so requiring perfection is maybe not that useful (and the dust particle is not visible in normal use, so it wouldn't matter even if it were jewelry).
No-one can give you certainty in that your watch cannot have faults. And that would be the case even if the watch looked perfect from the outside. But that's what the warranty is for. And as you (and I) have experienced, Garmin warranty repair/exchange works well.
And if you want more proof in that the dust particle won't affect HR readings: look at dcrainmaker's watches in his video where he describes the new ecg feature: the back glass of the watch is heavily scratched, and still those are clearly the watches he uses daily. 
