Hello Sir,
Thanks for your feedback.  I am grateful for HTTrack, I've only used it a
short while, some day if I have money, I hope to support their product.  
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I wish I had a definitive answer for you, but I present a "potential
solution," to restart your mirror, and this comment is merely to stand in
solidarity with you, but does not solve the "Purge issue." The same situation
happened to me (using Ver 3.48-19), and I am confounded by it as well, and
further warn, (be aware that it could happen again during the following
circumstances I will describe below).
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In my case, (it takes 3 days to download my website from my host). I set up my
filters to mirror one of my websites every few months, (I look for dead links,
and malware scan it, and evaluate it offline etc.).  On 3-1-2016 my website
was almost completely downloaded... I fell asleep, woke up the next day to
find that Windows-10, had modified my update settings (via the prior "Twitter"
update) installed a new update without me knowing, and restarted my computer
without alerting me.  After restarting I attempted to resume HTTrack, it
immediately "Purged" most of my mirror!
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I had a lot of trouble getting it to mirror the website again, some unknown
connection failures, I restarted my PC and network again (didn't help).  I
downloaded Ver 3.48-21, and started a new mirror of the website, which still
would not connect (multiple failures - some unknown conflict with the index,
but no server loop issues).  
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PARTIAL SOLUTION: The 11th attempt was a charm, I left all default settings
the same, but modified the MIME entry, I selected the last option to associate
all PHP, CGI etc, with Text/html.
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AFTER THE DOWNLOAD PROCESS RESUMED, I ASSUME AT THIS POINT THE FAULT IS 99% MY
OWN.  Either I modified the filters too much (to eliminate junk), or my PC
needed to be restarted yet again, or it wouldn't let me cache the whole
website, or my index has a conflict, or a webpage on my website needed a MIME
association, or restarting while HTTrack was going corrupted something.
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WHEREAS THE HTTRACK RESUMPTION "PURGE" WAS A SHOCKER, (and seemed to be
perhaps an abnormal default setting???)...and the rescanning of my site versus
the new mirror took 18 hours to resume downloading once again, (but it started
downloading again eventually)...by that time, I had started a new mirror with
Ver 3.48-21 on a separate computer, and I will compare the two mirrors, to
evaluate the results in a couple days when both mirrors are complete.
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Hope this helps, have a great day!