I've seen issues such as yours where a replacement gas tank grommet would leak usually because the OEM nipple or adapter going through the new replacement grommet on some lawn tractor tanks was not quite large enough for the new grommet so as to get a good snug friction fit. I can usually find a slightly larger hose barb nipple to get a good snug fit. Not a fun thing for something that should be simple fix and replacement grommet just not quite correct.
Seems strange that the replacement grommets that are suppose to be same as the OEM, but it happens. Seems they (probably China) get mm and inches sizes slightly wrong, just enough that a slow leak develops and not a fun thing on rear mounted lawn tractor tanks where the whole back end of the tractor has to be removed to get the tank removed AGAIN later. Tanks is good, just a wimpy grommet not correct for the OEM fuel line.