Tried on Win XP SP2, with heavily modified (through haltdefs) 6.2, no script 6.2 and no script 6.17
Steps to reproduce:
1. /join #.test
3. Observe that #.test is not shown in the channel list.
If you do raw debug logs or sniff the connection, you'll see that the channel name is transmitted, but mIRC does not display it in the Channel List.
Any channel in the form #.* (wildcards) are not shown. This includes #.#, #.423, #.3d.