I entered employment income of $1066 but have been reassessed by that amount because it did not show up as part of my total income. Why would that be?

It could easily depend on how and where you entered it.  You can add income via T4, T5, T4A, or income not on a Tslip, which are all in the income section.  If you just plugged it into a calculated cell, it may not have worked.

This is a good answer, but does not apply in this situation. I submitted it as a T4 in the income section, so it should have been calculated. Considering that, why else might I have been reassessed because the total income as determined by TurboTax was incorrect?

I can't hazard a guess why, then.  Your entry should have been perfectly okay. There have been some failures of the save function, but why that would be, I don't know.  I can give you the link for support if you like.
https://support.turbotax.intuit.ca/contact/