Tested using PHP 7.0.19 and do see the replacement but without the spacing you have.
Getting URLs like index.php?option=com_content&view=article&id=8&Itemid=116 which works.
Looks like & is kept in modules for example latest articles which doesnt replace & with & in the Url generated.

No the space is only here (in this topic) so the & amp; is not displayed here as a single &. I should have used code tags.
I get
index.php?option=com_content&view=categories&id=50&Itemid=288
instead of
index.php?option=com_content&view=categories&id=50&Itemid=288

There were changes to ampReplace in Joomla 3.7.1

a7705bf#diff-17d077571f8b5fe34c3d9f7fec9a6e44

@wilsonge @mbabker The framework merge has broke this ...

try removing htmlspecialchars in the modules/mod_menu/tmpl/default_component.php file on line 52 to make that line:

echo JHtml::_('link', JFilterOutput::ampReplace($item->flink), $linktype, $attributes);

that fixes the issue I think

Give us a unit test case on the repo to work with then. joomla-framework/filter#22 didn't break the lone test case so if it can be proven that change broke it that will help us to fix it.

Give us a unit test case on the repo to work with then

Actually if you check I did actually contrib several more tests on this exact merge!

Im unclear what the exact problem is here, but the framework change broke joomla, but im not sure if thats cause joomla code was crap and the framework was not...

After being shot down repeatedly by certain senior project member telling me that unit tests are ******* Im in no rush to get out of bed and write more

Im thinking this is joomla, not the new ampReplace, as the input to the ampReplace method is:

index.php?option=com_content&view=article&id=1&Itemid=101

which is clearly wrong, so my suggestion here is probably right

Note to self, merge additional tests up...

Please test #16089 and see if that fixes your problem

These were the only 2 cases I could see where a htmlspecialchars was running before the ampReplace

I confirm that, as stated PhilETaylor, removing the htmlspecialchars from
modules/mod_menu/tmpl/default_component.php
and
modules/mod_menu/tmpl/default_url.php (for non absolut external url links)
fixes the issue.

Problem is also solved by simply undoing the change to OutputFilter::ampreplace

preg_replace parameters look wrong

Problem is also solved by simply undoing the change to OutputFilter::ampreplace

No that is the incorrect way forward. A step back.

The preg_replace is fine.

Set to "closed" on behalf of @franz-wohlkoenig by The JTracker Application at issues.joomla.org/joomla-cms/16088

closed as having PR #16089