It's a Trends & Tendencies thing...
Cells that are approaching max electrolyte density will bubble. They will still bubble even at .5 amp charge rate.
The trick is to note your voltmeter and ammeter readings when the cells are all bubbling. Then you can use the meters instead of staring into the cells all the time.
A few people here went nuts when I said a hydrometer is needed to determine when cells are fully charged. Shame. When cells are fully charged, note the bubbling. Then try using the meters for awhile. After a month, go back and re-check with a hydrometer and see if your hypothesis remains valid.
Simple and easy. But a few folks think holding a hydrometer in hand is worse than getting a hypodermic needle jab in the ---.
It's merely a matter of getting to know your batteries. They have quirks. But they are easy to outwit. The hard thing to outwit is Smart Chargers. They suck you down to their level then beat you with tenacity and hyperbole.