MODEM AIS doesn't work as -B AIS

[emanuelelaface]

I have a Raspberry Pi with IQAudIO that works perfectly with direwolf (DEVELOPMENT version 1.8 D (Apr 26 2025)) for APRS. I tried also AIS and I noticed an issue that shouldn't exists according to the manual.

If I run it with -B AIS everything works properly.
If I run it with MODEM AIS it does not decode any AIS message, and in the log says

Channel 0: 39999 baud, AIS, +, 48000 sample rate x 4, Tx AX.25.
The ratio of audio samples per sec (48000) to data rate in baud (39999) is 1.2
There is little hope of success with such a low ratio.  Use a higher sample rate.

while when I run it with -B AIS (with the exactly the same configuration file) I have:

Channel 0: 9600 baud, AIS, +, 48000 sample rate x 3, Tx AX.25.
The ratio of audio samples per sec (48000) to data rate in baud (9600) is 5.0

and everything works fine.
The configuration file is

ADEVICE plughw:Zero,0 direwolf_lowlat
ARATE 48000
ACHANNELS 1
CHANNEL   0
PTT GPIOD gpiochip0 -25
TXDELAY   40
TXTAIL    10
MODEM   AIS

I tried to change to MODEM 9600 and the ratio between sample rate and baud is then again fine (5.0) but the encoding is no longer AIS but K9NG/G3RUH and so it does not decode AIS messages.