It says "SHOULD" not "MUST", so the client can assume the server will keep its promises, it's just not recommended.
"SHOULD This word, or the adjective "RECOMMENDED", mean that there
may exist valid reasons in particular circumstances to ignore a
particular item, but the full implications must be understood and
carefully weighed before choosing a different course."
Yes, I am aware the RFC used should and not must, but nevertheless their recommendation is clear.
And of course if that item is to be ignored "the full implications must be understood and carefully weighed before choosing a different course." Here the implications are that the device may cause other devices to drop their connections, which is a pretty severe implication, imo.
Note that it doesn't say that such verification has to be done prior to recommencing use of the leased address - just that it should be done. Arguably then the described implementation follows the letter of the recommendation. Your witness, counsel.