Merging topics

Hey Maarten,

I've further refined my topic model but now I'm noticing a couple of weird issues.

If I merge topics (even a single topic into another single topic) my topic model appears to collapse from 32 topics to about 4 including the outliers. The same thing happens if I 'update_topics()' with the countVectoriser.

Another thing I've noticed is that if I set the min_topic_size hyper-parameter it does nothing even though I'm using HDBSCAN. Is that something you have seen before?

Cheers.

Start by reproducing the topic collapse after merging topics and after calling update_topics() with the countVectoriser, then separately check whether min_topic_size affects a model using HDBSCAN. The issue does not name files or tests; done would require isolating the cause of both reported behaviors and confirming the expected topic count and minimum-size behavior.

Written by the indexing model from the issue text.