FFFFFF padding included for some values in ScalaSig value in classfile

I'm having some problems with Proguard which nulls out the value of ScalaSig which result in the Scala compiler not accepting the file (reported error and waiting for answer).

But I wanted to ask if it's correct that some values should be padded with FFFFFF or if this is a bug? The responsible file could be ch.epfl.lamp.fjbg.JOtherAttribute (could not find source code).

Example:

ScalaSig: length = 0x41F 
04 01 FFFFFF81 05 06 08 01 02 FFFFFF84 FFFFFF80 FFFFFF80 FFFFFF80 00 09 02 13 
53 75 62 64 69 76 69 73 69 6F 6E 49 74 65 72 61 
74 6F 72 0A 02 03 04 01 09 64 61 74 61 74 79 70 
65 73 0A 02 05 06 01 0B 66 69 6C 74 65 72 67 72 
61 70 68 0A 01 07 01 09 70 68 6F 74 6F 6E 69 73 
74 03 00 13 03 00 0A 12 10 02 0B 10 0D 01 0C 0A 
02 0D 0E 01 04 6C 61 6E 67 0A 01 0F 01 04 6A 61 
76 61 09 02 11 0C 02 06 4F 62 6A 65 63 74 10 02 
13 16 0D 01 14 0A 01 15 01 05 73 63 61 6C 61 09 
02 17 14 02 0B 53 63 61 6C 61 4F 62 6A 65 63 74