notmuch reindex wipes existing tags

[Franz Fellner]

Hello,

I setup notmuch for my dad specifically for mass-extracting
pdf newspapers he receives.
When the pdf was stored successfully the message gets the 
tag attachments-extracted added.
Today I wanted to go further and make use of afew specifically
for mail moving to free up space on the IMAP server.
I initially ignored the List-Id feature but now wanted to make
use of it. Configured notmuch appropriately, configured afew.
Ran notmuch reindex.
And now all custom tags were wiped, especially attachments-extracted.
I now can't see if the pdf of a certain message was already saved.
This is 3 years of 5 newspapers a weak. I must delete the files
and re-extract them.

I did not expect that, especially after reading the man page
which doesn't warn about resetting tags.

So here my question:
Is this a bug or a feature?

Regards
Franz