Chinese elms can be temperamental. We get lots of people writing in to say their Chinese elms are dropping leaves, including another one today. They are also a little confusing. In general there are temperate deciduous trees that drop their leaves in the fall and go dormant and tropical evergreen trees that keep their leaves and grow year around. Temperate deciduous trees need to stay outside in the winter and be allowed to go dormant. Tropical evergreens are better for indoor bonsai and keep their leaves and grow most of the year. However, Chinese elms are kind of in the middle, being a subtropical tree. Some people keep them as indoor bonsai and some people treat them more as temperate trees. Here's one statement:
Chinese Elm trees are very flexible about their environment. If adapted
properly, they can be grown indoors year-round. If hardened off to
the cold gradually, they can drop their leaves in the fall and be
considered deciduous trees, so you have a few options. However, it is
important to find out how your Elm has been growing recently. Some
Elms come from southern China, and have never experienced cold
conditions. It would be dangerous to keep these Elms too cold the
first year or two. Other Elms have gone to freezing and dropped their
leaves for many years; as a result, these Elms may not be too happy
indoors for the first year or two. If you can not determine where your
Elm has been, then keep it outside for the summer and bring it inside
for the winter. You can bring it in when the temperatures are around
50Â°F. If kept indoors for the winter, a cooler location is preferable
(50Â°Fâ€“65Â°F). Chinese Elms may drop some leaves in the winter due
to the natural decrease in light. This is normal.
Two things about yours particularly. You said light is not a problem, but you also said it is by a NORTH facing window. North facing windows get the least light in winter (unless you are in the southern hemisphere). South facing windows are best. So I think with shorter darker days and the North window, it is being triggered in to dormancy.
Also your soil looks very organic and moisture holding, more like potting soil than bonsai soil. Bonsai soil should be very mineral and loose and free draining. Especially since it is going dormant, now is not the right time to repot it. In the early spring you should repot it in to good bonsai soil.
Since yours has been indoors, I would not put it outside at this point, but I would give it a semi-dormancy. Put it in the coldest place you have in your house, do NOT fertilize, water very little, don't worry about the light. It will finish dropping leaves and be dormant and rest for awhile. Then in late winter once days are noticeably longer, bring it back to warmth and light and more frequent watering. Once it is starting to grow again repot and start fertilizing.
Best Wishes! Keep us posted how it is going for you. Welcome to the Forum!