What does the repeated message "No further splits with positive gain, best gain: -inf" mean?
I don't know the internal specifics about LGB behavior in this case but I don't think it is stuck in an infinite loop.
From Github's LightGBM issues we can find that:
What's the meaning of &quot;No further splits with positive gain, best gain: -inf&quot; message?

It means the learning of tree in current iteration should be stop, due
to cannot split any more. I think this is caused by
&quot;min_data_in_leaf&quot;:1000, you can set it to a smaller value.

That is, LGB is trying to split the data in leaf but it can't.
The greater min_data_in_leaf is the more conservative the algorithm is.
Its optimal value depends on the number of training samples and num_leaves.