decremental backup

i've got two existing full backups and the command i'm using in my
script is to make the decremental is:
dar -+ "../${prev}-decr" --delta sig --ref="../${prev}-full"
--aux="../${current}-full" --hash=sha512 --slice=2G
--overwriting-policy Pp --alter=decremental --alter=keep-compressed

however interactively i'm asked:

EA and FSA of file <ROOT>/siteinfo-namespaces.json.gz-sha1sum.txt are
about to be dropped from the archive and marked as already saved,
proceed? [return = YES | Esc = NO]

repeatedly for every file


i can add the -w switch so it stops asking interactively, however this
results in the resulting archive being roughly the same size as the
aux archive

i know the differental backups work incrementally with much smaller
resulting archives, however i want to do decremental - what am i doing
wrong? i've tried various different overwriting policies with no
impact on the result


just fyi, dar -V is:

 dar version 2.7.1, Copyright (C) 2002-2021 Denis Corbin
   Long options support         : YES

 Using libdar 6.4.0 built with compilation time options:
   gzip compression (libz)      : YES
   bzip2 compression (libbzip2) : YES
   lzo compression (liblzo2)    : YES
   xz compression (liblzma)     : YES
   zstd compression (libzstd)   : YES
   lz4 compression (liblz4)     : YES
   Strong encryption (libgcrypt): YES
   Public key ciphers (gpgme)   : YES
   Extended Attributes support  : YES
   Large files support (> 2GB)  : YES
   ext2fs NODUMP flag support   : YES
   Integer size used            : 64 bits
   Thread safe support          : YES
   Furtive read mode support    : YES
   Linux ext2/3/4 FSA support   : YES
   Mac OS X HFS+ FSA support    : NO
   Linux statx() support        : YES
   Detected system/CPU endian   : little
   Posix fadvise support        : YES
   Large dir. speed optimi.     : YES
   Timestamp read accuracy      : 1 nanosecond
   Timestamp write accuracy     : 1 nanosecond
   Restores dates of symlinks   : YES
   Multiple threads (libthreads): YES (1.3.2)
   Delta compression (librsync) : YES
   Remote repository (libcurl)  : YES
   argon2 hashing (libargon2)   : YES

 compiled the Aug 17 2021 with GNUC version 10.2.1 20210110
 dar is part of the Disk ARchive suite (Release 2.7.1)
 dar comes with ABSOLUTELY NO WARRANTY; for details
 type `dar -W'. This is free software, and you are welcome
 to redistribute it under certain conditions; type `dar -L | more'
 for details.