Here is what TIMATIC says. It is what the KL agent would have seen

Passports and passport replacing documents of all visitors
must contain at least 1 unused visa page for Visa
Administration Sticker (VAS), stamps, etc. Please note that
this does not include the page reserved for Observations
or "Amendments and Endorsements".

OP played it too cute. Even though it may be that Tanzania did not actually need a full page, it requires one to have a full page. Doesn't matter that OP already had his visa.

Don't know why KL allowed OP to fly the next day. Perhaps the second agent misread, someone called and learned that they don't enforce the requirement or whatever.

But, OP wasn't improperly denied boarding. So, no compensation for anything under EC 261/2004 or otherwise.