Somewhere between a bug or feature:

[Han]

The where function | and the subst() function handle substitutions differently. The latter will resolve variables (and it would seem only one level deep) prior to substitution whereas the former does the reverse.

x:=a^2+1;
subst(x+1,x=1);
(x+1)|(x=2);
subst(x+1,a=1);
(x+1)|(a=1);

The only exception is the one pointed out by parisse earlier in the thread.