Has "extra" bounties been a recurring event in the past?
As others have mentioned, bounties that reward existing answers are explicitly encouraged by the design, and bounties have had that explicit encouragement designed into them since the bounty system reached its final shape in late 2011.
The use of such bounties has been in place in this site for the past seven years or so: you can find all of those bounties in this Data Explorer query, which can also be used (as in this query) to find the rate at which they've been posted. Graphically, it looks like this:
(Date on the horizontal axis, accumulated number of 'reward existing answer' bounties on the vertical axis.) There is a minor bump in the rate over the past ~month or so, but it does not represent a break with the long-term trend, and it is not the largest localized burst in the rate historically.
This represents, at current SEDE count, 192 out of a total of 2920 bounties (full listing here, rate graph here). (Also, for full disclosure, about a quarter of those 192 were set by me; as I've argued previously, we need more of those bounties (and bounties overall), not less.)
To comment on your final set of assertions:
Essentially, I am concerned that prolonged use of the bounty system in this way will overjustify the creation of "exemplary" answers
How exactly is promoting quality content a problem, again?
and eventually degrade the quality of the site in one of two ways (or both): The answerer, in attempt to get reputation/recognition, is verbose at best
If you think this is a problem, show it. Show evidence of answerers being "verbose at best", to the detriment of the content, in an attempt to fish for post-answer bounty rewards. (Which, frankly, as a strategy, makes no sense at all. The answers being rewarded are not being prized for being verbose, but for being high-quality.)
OR the bounty system loses its appeal as users find there is nothing to answer.
From the 192 'reward existing answer' bounties listed in the query above, a full 53 of them (i.e. 25%) attracted an additional total of 70 answers while the bounty was running. Given that these were questions that already had quality answers, this seems like it's still working to increase contact between answerers and interesting questions (which have a high correlation with the exemplary answers), thus producing more quality answers.
If you think this is a problem, then show it.