Is visa issuance guaranteed after COE?

Hello everyone. Is it possible for visa to get denied even if COE is already issued?
If it is, what are the possible reasons for it?

Once you have your COE if your application was done right and you are actually eligible for entry then it is pretty safe to assume with a high degree of confidence that you will get your visa. But no, it's never guaranteed until you have it in your hands. Applications are judged on an individual basis and there are a million possible reasons for denial. It could happen that the issuing embassy picks up on something that wasn't caught while the COE was being processed. If you are not eligible for the visa for whatever reason - say you are a criminal wanted by Interpol to go with an extreme example - you don't suddenly because eligible just because you managed to get them to (mistakenly) issue a COE for you. In this case the embassy would not issue the visa.

Are you eligible for entry/the type of visa you are applying for? If yes then probably nothing to worry about. If no then... well good luck not getting caught.

Its never guaranteed. You can even read that in your CoE it should be written down that this paper does not guarantee you to enter Japan. They can deny even if any circumstances changes. They can find maybe something what you tried to hide. So, if you have everything done perfectly and there is nothing illegally done, then it should be okay.