Sieve regex behavior

Hi,

I'm having troubles lately with a Sieve script of mine:

 require "fileinto";
 require "mailbox";
 require "regex";

 if header :regex "Subject" "\\[.*PATCH.*\\]" {
     fileinto :create "Inbox.Patches";
 }

This should move all patches with "[PATCH]" or "[RFC PATCH v3]" into this Inbox subfolder, I'm pretty sure this version worked flawlessly in the past but since a couple of weeks ago it's not working with some emails.

Debugging this, with manually editing mail files, I came to this discovery. That one mail matches and it's getting moved to the correct folder:

 Subject: Re: [PATCH v2 3/6] arm64: dts: qcom: qcm6490-shift-otter: A

Adding one more character leads to it not getting moved:

 Subject: Re: [PATCH v2 3/6] arm64: dts: qcom: qcm6490-shift-otter: 

Ad

Does anybody have an explanation for this why this broke, and how to fix either Dovecot/Pigeonhole or my Sieve script?

I've already lost hours trying to figure this out and I'm not getting any wiser...

Regards Luca