Calibre remembers the number even when you change the name. It does use two different edit boxes for series name and series number, so I'm not really surprised.
My expectation is that you will only get [5] if 1-4 already exist in that series. But I've exhausted my ability to care.
I was going to ask if you have calibre_series_meta:true, but that feature outright breaks with these settings and will be fixed in the next test version.
So congratulations, you have yet again found a new corner of the envelope to push that no one else has.
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