I haven’t used your specific filter but most canister filters require a little air purging as the system gets going again after being cleaned or setup. Usually just a little rocking back and forth gets it done.
Lots of it depends on what time frame you are talking about. If the bubbles are coming out 5 minutes after a restart, no big deal but if they are coming out 2 days later, look for a leak letting air in.
Another point to consider is what amount of air you are seeing. A small amount of air oleft in the can due to poor restart should give more bubbles which then reduce as the amount of air is fed out, but if the same amount of air is coming out at first as there is after 10 minutes there is a problem.
Something like a leak around a seal at the canister top is one to check. A very thin coat of petroleum jelly ( aka Vaseoline?) is almost always good as it helps them stay flexible as well as slide better.
For those still thinking it's oil, do some homework!