Question about how SVN works, specifically the svn log command?

Question

linux

There is an SVN server with a bunch of repositories and various branches.
The question is when you run the command:

svn log -r 5555:5560 svn://svnserver/path-to-repository/sub-folder/

Then we will see something like:
- r5555 | username | data | 1 line
comment
- r5556 | username | data | 1 line
comment
- r5558 | username | data | 1 line
comment
- r5559 | username | data | 1 line
comment
- r5560 | username | data | 1 line
comment
Revision c numbers 5557 disappeared from the output.
But if you run the command:

svn log -r 5557 svn://svnserver/path-to-repository/sub-folder/

Then we will see this revision:
- r5557 | username | data | 1 line
comment
Why is this possible? There is a possibility that this may somehow be related to the fact that the branch is merged (merge)
# svnserve --version
svnserve, version 1.5.7 (r36142)

Answer 1

In general, this is most likely a bug in the SVN server.
We tested the same thing on a Read-only copy of the server with version 1.7.4, everything is fine, all revisions are shown.

Answer 2

Also, if the sub-folder contains something connected via externals, then its history will also not be shown when requesting a log for the subfolder