Can a Reward for Healing Transform from a Positive $ Dollar Value to Nothing at Zero?

Yes, it can happen, but it is not very common. For example, if someone downvotes the post strongly after you've made your vote. Depending on the used SP for the downvote it could bring the payout amount to zero.

There are different factors that constantly influence the exact payout and curation amounts (every few seconds):

• Current size of the reward pool (System Info -> Total Reward Fund)
• Median STEEM/SBD market price (Market Info -> Internal Median Price)
• STEEM per vesting share value (Market Info -> STEEM Per MVests)
• Other votes on the same post

I don't see any problem in your case. The 'pending curation' shows zero, because you already got paid the curation reward and therefore it is no longer pending:

I hope this helps :)