The update process of the haproxy addon to Version 1.8.14 (IPFire 2.21 Core Update 124 --> 125) overwrites the configuration file /etc/haproxy/haproxy.cfg with the default config file of the addon package. There ist no further notice about this nor any warning during the update process. At least one user was able to reproduce this behavior (earlier this year at core update Nr. 119): https://forum.ipfire.org/viewtopic.php?f=27&t=20485&hilit=haproxy
Interesting that this never was an issue before, but you are right, there is no backup being created: https://git.ipfire.org/?p=ipfire-2.x.git;a=commitdiff;h=c519be42262c629abac86fb251a3f3921d42310d Interestingly there was a backup include file which listed the configuration file that should have been archived.
Your patch fixed it.