aboot unzip "unknown compression method" on recent (post 6.4) kernels?

Hi,

     I have been unable to boot kernels after 6.4; aboot fails with 
"unzip: unknown compression method". Specifically:

         - vmlinuz-6.4.0-1-alpha-generic --> works
         - vmlinuz-6.6.15-alpha-generic --> fails
         - vmlinuz-6.11.6-alpha-generic --> fails

     This is on a XP1000 (EV67/Tsunami/Monet). aboot is 
1.0~pre20200212-1. FWIW, "gzip -l --verbose" says all kernels are 
compressed with the same method:

$gzip -l --verbose vmlinuz-6*
method  crc     date  time           compressed uncompressed  ratio 
uncompressed_name
defla 2ad47e09 Nov  4 13:57             7010635 24244720  71.1% 
vmlinuz-6.11.6-alpha-generic
defla 148fa7ac Jul 22 23:53             6814008 23614088  71.1% 
vmlinuz-6.4.0-1-alpha-generic
defla 30836626 Feb  4 12:28             7045662 24145360  70.8% 
vmlinuz-6.6.15-alpha-generic

     Did something more subtle change in compression in between kernel 
6.4 and 6.6?

                     Thanks,

                     -Alex