I confirm the problem, thanks! Will be fixing this.

 

I confirm. Thanks Chris!

Here is how it works:
All tags are tried to find out which should be suggested.
1) First, tag beginning is matched, verbatim
2) If 1 fails, tag is split into parts treating any non-alpha symbol as a separator. Then all resulting parts (which are alphanumeric) are matched from the beginning.

Example:
All tags: #tag1, tag2/subtag, tag3/#subtag
You have entered: #
 #tag1 is tried - tag beginning matches => #tag1 is suggested
 tag2/subtag is tried - beginning doesn't match, splitting gives ['tag2', 'subtag']. From these parts, none match.
 tag3/#subtag - begginning doesn't match, splitting gives ['tag3', 'subtag'], also no match.

So as you can see, the behavior you are observing is explained by the algorithm.
I think the algorithm could be improved introducing a two-stage split: first, split by '/' and match all child tags, then, split by punctuation.