Polynomial partial substitution errors out

[user202729]

Steps To Reproduce

R.<a,b>=PolynomialRing(QQ)
S.<c,d>=PolynomialRing(QQ)
(a+b).subs(b=d)  # justifiably error out
b.subs(b=d)  # works fine

R.<a,b>=PolynomialRing(QQ, implementation="generic")
S.<c,d>=PolynomialRing(QQ, implementation="generic")
b.subs(b=d)  # error??
a.subs(a=c)  # funnily enough this works fine

Expected Behavior

the error?? example should just work

Actual Behavior

see comment

Additional Information

No response

Environment

• OS: Linux
• Sage Version: latest

Checklist

• I have searched the existing issues for a bug report that matches the one I want to file, without success.
• I have read the documentation and troubleshoot guide