I have found similar issues with trying to maintain small motorcycle batteries. I have had the best luck when using a very small .8 to 1amp "wall wart" maintainer. The larger ones, no matter how fancy or expensive, seem to have trouble not boiling the electrolyte on a small 12 to 14 ah battery, even though they claim compatibility and have a low amp setting.
Deltran and Battery Saver (save a battery) all make smaller 750 ma to 800 ma chargers (aka 10 watt) suitable for motorcycle batteries.
Examples:
https://www.batterysaver.com/store/p102/1000.html
www.batterytender.com
I have a lithium battery on my motorcycle now and use a deltran wall wart lithium charger.
Got tired of changing flooded batteries every three years.