First, for a baseline, I have checked and the problem does not manifest in
  BusyBox v1.2.1 (2006.09.04-14:45+0000) multi-call binary

In the interfaces file, for the 'netmask' parameter (but not address or gateway parms!) if there are two or more whitespace chars (tab & space tested) between the netmask token and the parameter then I get the following error:
=-=-=-=-=-=-=-=-
  Reconfiguring network interfaces: Error: an inet prefix is expected rather than "172.16.1.254/4294967295".
=-=-=-=-=-=-=-=-

Sample problem config (all whitespace below are simple SPACE chars; single spaces except for preceding the netmask parm):
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
auto lo
iface lo inet loopback

auto eth0
iface eth0 inet static
address 172.16.1.254
netmask  255.255.255.0
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-

Workaround is obviously to reduce whitespace after the netmask parm, to be only a single character of whitespace.

Thx for BB!