Subversion behaving weirdly behind reverse proxy - iis

Question

I'm trying to migrate a Subversion server to a different machine, which is also already running IIS. Thus, I configured Subversion to run at port 81, and set up an IIS rewrite rule based on the host name:
<?xml version="1.0" encoding="UTF-8"?>
<configuration>
<system.webServer>
<rewrite>
<rules>
<rule name="svn.foo.example" enabled="true" stopProcessing="true">
<match url="(.*)" />
<conditions logicalGrouping="MatchAll">
<add input="{HTTP_HOST}" pattern="^svn.foo.example$" />
</conditions>
<action type="Rewrite" url="https://svn.foo.example:81/{R:1}" />
</rule>
</rules>
</rewrite>
</system.webServer>
</configuration>
In addition, "server proxy settings" are configured as follows (I think I left all of these as defaults):
enable
version pass through
keep alive
120-second timeout
reverse rewrite host in response headers
preserve IP in X-Forwarded-For
(The SVN server is VisualSVN 4.3.4; the IIS is Windows Server 2019 Version 1809.)
Where things get strange is that some stuff works, including on existing working copies. For example, I can:
use an existing working copy
add a file to that folder, and commit it
check that file's log
However:
use an existing working copy
check the log of a folder // works!
check the log of an existing file // does not work!
The latter fails with E160013: File not found. Indeed, if I look in the IIS logs, it returns 404.
If I instead (locally) check out that same file without the reverse proxy, I can get its log, so presumably, it's a problem with the proxying. But then why does it work if I add new files and check their logs?