So the memory leak indeed seemed to be coming from the frequent health check.
I customized the health check path and returned a simple response object, and it seems to do the trick.
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I still think I have something incorrectly handled somewhere and if the website would have a lot of traffic, the issue would probably resurface.
But it's not very likely, so I'll stick to YAGNI. (I'll keep an eye on the stats in any case.)
Thanks for the help.