Just a thought, if you apply heat and it gets the adjacent lead
joint(s) hot enough to melt...

Have been replacing old (1965) cast iron/oakum-lead jointed pipes in
our townhouse over last 10 years as they start to leak at the joints,
using new ABS and rubber couplings, also have rubber caps over the
major cleanouts which get cleaned out at least annually (tree roots).
No leakage or stoppage problems so far.