It's interesting that you hear increased bass as a result of a potential breach of seal, but maybe not necessarily to be unexpected given how the algorithms similar to Adaptive EQ
some other ANC headphones use behave when there is a breach of seal. Here for example the XM3 :
https://diyaudioheaven.wordpress.com/headphones/measurements/brands-st-x/wh-1000x-m3/
The "knee" beyond which the bass response is lowered is actually higher than with a proper seal. Perhaps something similar is happening in your case ?
The Bose QC35 II seems to handle that "knee" better when seal is breached :
https://diyaudioheaven.wordpress.com/headphones/measurements/bose/qc35-ii/
BTW that capacity to maintain a fairly decent FR target in the lower frequencies even with some breach of seal is one of the advantages of
some ANC headphones vs closed backs with no feedback mechanism and potentially solves the sort of sealing problems that plague headphones such as the K371. Obviously given this discussion,
potentially.