Definitely could be a bad o-ring issue, but the tank comes with no spare o-rings (except for a few drip tip o-ring replacements), and the tank really isn't a design that can be fully opened up by the end user to make o-ring replacements on. There is maybe only one user replaceable o-ring located on the underside of the tank. I generally lube that o-ring up a bit with some juice, as well as the one that is located on the bottom of my replacement coils, but that is all I have access to.
So, it would be hard to check if your issue is related to a bad o-ring on this tank. I would more likely suspect the issue is a bad coil. Factory coil heads tend to be a bit inconsistent, and sometimes just flat out bad once and awhile, so this would not be a surprise. Nautilus coil heads got so inconsistent and bad over time that I just stopped using Aspire coil head using tanks a few years ago, and never looked back.
Innokin factory coil heads are generally pretty good, for what they are, but I'm sure they still send out some bad ones sometimes. Switch out the coil head and see how that works for you. If that doesn't help, you likely just have a bad tank, and there is not much you can do about that other than try to have the supplier replace it - which can be a hit or miss proposition, depending on the supplier. Try contacting Innokin directly as well, if your more local supplier is not willing to be helpful.
There is also a small chance that you are not installing your factory coil head properly in the device, or that it was improperly installed by the factory in the first place, so check on that possibility as well. Make sure the coil is inserted the right way, and that everything is fully screwed down after installing. When opening the top of the tank to refill, or when adjusting the airflow, it is possible that the tank can unscrew from itself a bit at the bottom - which could certainly lead to a leak issue.
I haven't had any leak issues at all with my two 4ml Zeniths, but I can tell that some of my factory coil heads (0.5 ohm) are inconsistent in how tight or loose the wick material is packed. That's a common, almost unavoidable, issue with factory coil heads, and one of the reasons why some of us rebuild and wick our own coils instead. That's what I used to do, using various MTL RTAs, but that gets old sometimes (for me anyway) and sometimes I just don't have the time to mess with all that (though it usually isn't really all that much of an effort).
I have had a lot of luck with the Zenith tank, so it has become my go-to vaping tank. I hardly use my MTL RTAs at all anymore. I use high VG (70/30 VG/PG) juices, but I almost always thin those juices out a bit with distilled water. How much I do that changes a bit depending on the coils I have in my Zeniths. Usually 15 drops of distilled water per 15ml of juice works well, but sometimes I have to up that ratio a bit (by another 5-10 drops or so of distilled water) if the particular coil I'm using feels like it runs a bit dry. A few other Zenith coils I've had run a bit "wet", and for those I sometimes don't add any distilled water at all to my juice. I wish the coils were more consistently wicked, so I would not have to mess around like that sometimes, but despite that I haven't had any real dry hit or gurgle issues with these tanks - which is one of the reason I like them, and so highly recommend them.